They are Ethnic Chinese who were operating scam centers in collaboration with junta at northern area Laukkai.

There are more at shwe Koko area.

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.