Dubious, it is more or less impossible to measure this since religious identification is so superfluous, syncretic, and ambiguous in many places (I'm looking at you East Asia).

I mean, China is probably dark because there was probably an even amount of self-reported Buddhists, Taoists, Confucian-ist?, and Folk religious practitioners according to the census. But in reality, historically in China, all of these traditions have been practiced simultaneously among the Chinese population; syncreticism is the norm, not the exception. Belief, membership, and "being the absolute truth" is not emphasized at all like it is in Abrahamic faiths and the majority of people don't even believe in any of it but still practice the rituals for cultural reasons. (Eastern traditions are orthopraxy rather than orthodoxy (right practice rather than right belief), it doesn't matter if you believe in the dogma/doctrine, as so long as you go through the proper motions and say/do the right things at the right time, you're good).

Also, a direct translation for the English "religion" did not exist in Chinese. Instead, early missionaries decided to translate the word 宗教 as religion but really, it means something along the lines of "teachings of the elders." So Confucianism, despite it not having any doctrine or dogma regarding the supernatural world, the origins of the universe, or the of the "big questions," is a 宗教 but Chinese folk traditions, which does have mythologies, supernatural beings, and explanations for the workings of the universe, is not a 宗教. The supernatural aspects of Confucianism, such as ancestor worship, 天 ("heaven"), and worshiping/honoring Confucius himself is not of Confucian origin but already existed as part of Chinese folk religion(s). Also, it is important to note that 宗教 refers to the "big 3" (Buddhism, Confucianism, Taoism) since they are mostly organized and institutionalized religions with a a founder and a set of texts and official teachings. Chinese Folk Religion (or as I like to say, transitions) are none of those. They have been practiced by the ethnic Han since before written history and don't have any organized structure, teachings, doctrine, dogma, founders, or even standardization. They are simply a collection of stories and mythologies that describe supernatural beings and legendary events as well as certain metaphysical concepts. Because it is unorganized and based solely on oral tradition, the same story or characteristics of a supernatural may vary from place to place ad time to time. In fact, there are usually competing stories or descriptions that may contradict one another (I know for one that there is more than 1 creation myth and that dragons don't have a single unified description of appearance/power). But like I said before, beliefs don't matter that much and most people don't take these stories seriously either. Also, almost no one would call themselves "a follower of the Chinese Folk religion" since that concept doesn't even exist and yet at that person may practice certain rituals or have shrines in their house or shop.

Some beings such as Guan Yu can simultaneously be part of the Folk/Taoist pantheon and a Bodhisattva, also Guan Yu was nothing more than a normal human and had a documented birthplace, hometown, etc. just like everyone else. Yet he is worshiped/honored as a 神 (sometimes translated as god, but is closer to the English word 'spirit' since anything from the soul of your dead grandma to powerful beings who control the weather is a 神). In fact, most of the most popular 神 were just normal people (generals, emperors, scholars, even humble maidens) who became exalted after their death. Also, many Bodhisattvas are equated with pre-existing Chinese 神 (such as Avalokiteśvara and Guanyin) or earlier concepts with later concepts (such as 上帝 and 天).

The lay person casually practiced each tradition, calling on the appropriate scholar/priest/soothsayer/shaman/monk for advice (Buddhist focused more on the afterlife and the big questions of the universe, shamans/soothsayers basically dealt with luck and were basically fortune tellers, Taoists managed the temples of the 神 and our relationship to nature, Confucian scholars were the scribes that wrote down history and government documents but weren't really "priests" since Confucianism is more of a philosophy, though Confucians were commonly teachers and taught their philosophy [which was the official philosophy of the state throughout most of history]). Worship was a public thing, not a private relationship with some deity. Often entire families or communities came together at the local temple (most of the time not really specified as Buddhist or Taosist or such but just the general temple) and worshiped/honored any number of local protector 神, sometimes the Buddha or Confucius or another important historical figure (in the context of being a 神 though). And also in the home a shrine dedicated to one's ancestors, some places even have a "clan house" where people of a common last name come together and honor/worship the (often legendary/mythological) progenitor of the clan/lineage. Also, the Emperor had a special job since only he could mediate the relationship between 天/上帝 (the closest thing they had to a supreme being, all-seeing being) and the human world.

It is only in modern times that foreign influence modern Chinese people have ideas closer to Western thought, it also doesn't help that China is officially communist and purged all regions and cultural expressions during the deadly Cultural revolution. Thus, traditional Chinese religious concepts have been tainted for most of the population who often use traditional Chinese terms with the Western interpretations. Most of the time however they're completely atheist. Traditional Chinese religiosity however is stronger/more authentic in Taiwan and other non-communist Chinese communities because of this. Japan more or less has the same situation but with a syncreticism between Buddhism and Shinto (which is the Japanese folk/ethnic religion/tradition) though there is a strong influence from China and many Chinese figures and concepts (such as Confucianism) are found in Japan. Vietnam and Korea too but they are closer to the Chinese meta than Japan and their folk religions overlap with Chinese folk traditions/Taoism/Confucianism for the most part.

TL;DR The Chinese religious/spiritual situation is complicated and comparing it to Western religions is like comparing apples and oranges. In essence, the Western religious/spiritual meta is like a salad where all the components live together but remain separate, and the East Asian religious/spiritual meta is like one giant melting pot where any religious tradition or teaching becomes popular long enough in Chinese history simply blends into all the other traditions until it's one indiscernible goop.