I have heard of multiple L2 Chinese speakers who apparently sound completely native, but zero L2 Japanese speakers who sound completely native
I mean, you have to take that with grain of salt, no? It's not the language you focused your life into, or try to find every little issue with every detail of speech in others with, while Japanese is.
If you look at your own, or any of the people who are said to have Native/near native level of Japanese, you will ALWAYS find comments from natives saying things along the lines of "if I closed my eyes" blah blah blah. Are you saying they're all wrong? Are you saying only you know better and they're just lying or being nice? If that's true, then couldn't it be completely possible they're all lying about Chinese too?
As a native English speaker it's quite hard to fool someone when you learned English later in life, they have tells, some words just come out wrong, even when 99+% of what they say is literally perfect, after a while, you can catch something. I'm CERTAIN those exist in every single language, and getting rid of them might literally be impossible without dropping your native language completely, or focusing on them so heavily that it becomes an actual obsession and most of your life is in that language with you caring about and somehow figuring out/paying attention to those times when you "mess up." AJATT and MIA could be said to be forms of obsession in this regard.
I would personally argue that people look far too much into this, and people from different parts of the country have weird hangups and mess ups, different levels of education and ability, and that pointing out these small tells or thinking they're overly important is akin to thinking someone who sounds "southern" in some of the words they say, is somehow not fluent in their language 100%. It's kind of like watching Kaz nitpick someone, but only ever point out using the "wrong" dialect, when he himself would absolutely do the same thing, given he's not from the Tokyo region, but we expect different of L2 learners for some reason.
I do think the Japanese language learning community is different than others, and I think especially on the top end, things are held to a much higher standard than in other languages, and perhaps this comes from the Japanese themselves doing much the same thing. I'm certain you could find Japanese people who would claim you could NEVER get to a "10" (or maybe even 9..) because Japanese is too hard, or some nonsense. The level of work people have to put in no doubt plays a role as well.
I think in the end there is a whole lot to be said about the standard placed on L2+ learners of Japanese. I find all too often things are pointed out as "wrong" "weird" something a Japanese would "never do" etc etc, but then you see Japanese people stutter, mess up, say the wrong thing, use the wrong term, or even use something in a way it's used now (sometimes or rarely), but used to be used differently, and instead, because they're Japanese it's "ohhhh, they did it in such a Japanese way" are quickly corrected and things continue, as opposed to being rated at an "8" or something. I've even seen this standard mistakenly used against people born and raised in Japan, who only speak Japanese, before people realized they had been, because they LOOKED foreign.
That's not exactly about accent, but when I say a word improperly in English, even to the point of someone not even understanding what the word was (and laughing at me), no one rates my native level of English lower, sometimes your brain just messes up. I would never rate my (or rather, someones) level of English based on my/their lowest mistake, but instead on their overall ability and overall consistency, much like I would rate even a native level speaker on their level of vocabulary, education, knowledge, etc etc.