• Aren't Vietnam and Thailand...closer to China than Indonesia?

He's using the word "Indonesia" but include Indonesia. I think he meant to use "Indochina" but got confused about the word.

But it would probably make more sense to group Thailand at least with Myanmar, Laos and Cambodia than China.

Vietnam you could debate but historically there has been fairly large territory exchanges in that area that didn't involve China; much of present day Thailand and Laos were part of the Khmer Empire in the Middle Ages while Cambodia and Laos came under Siamese rule in the modern era before being lost to the French from Vietnam. Siam and Burma have also been invading and trading territory for centuries.

There are also cultural similarities, Burmese, Thai, Laotian and Khmer as well as minor languages such as Lanna (northern Thailand) all use Brahmic writing systems that evolved from India for example, and this is also true of historic scripts used in Malaysia and Indonesia. This is in contrast with Vietnam that used Chinese.

There are substantial common Indic influences, although these proceed down as far as Indonesia: both Thailand and Indonesia use Garuda, the mount of Lord Krishna, as their national emblems. The whole region has substantial Indic and Hindu history, Angkor Wat in Cambodia and Prambanan in Indonesia were both huge Hindu temple complexes that have commonalities with other sites throughout the region, while Phimai in Thailand was built by the Hindu Khmer, although as a Buddhist temple.

Again Vietnam you could debate, although in modern times it is in ASEAN and has a certain colonial common history with Laos and Cambodia... But if you were going for a very general grouping I think it would certainly make more sense to group Thailand, and arguably also Vietnam, in a modern context, with the rest of SE Asia, rather than China.