Was the Chinese annexation of the Second East Turkestan Republic "illegal"?

Jul 2019
1
Devon, United Kingdom
#1
Today, East Turkestan and Uyghur separatists claim that the occupation of their region by both the KMT and CCP, was "illegal" meaning that it should not be part of the modern PRC. I was wondering whether this argument had any merit?

I know that the Second East Turkestan Republic was a short lived Soviet puppet state but the circumstances of its absorption back into mainland China as an autonomous region are slightly unclear to me.