were they Chinese citizens?

They are usually Chinese triad gangsters who operate scam businesses and illegal casinos in south east asia.

https://en.wikipedia.org/wiki/Triad_(organised_crime)

China claims jurisdiction because 20 Chinese citizens were murdered, if that’s what you’re wondering.

They are Han Chinese, Most of those triad family come from Yunan borders a few decades ago. In Myanmar , thanks to military junta , they can easily buy citizenship by bribing authorities One of them is even a MP Senate in Military aligned party.