It doesn't matter whether there is JV in the past, we should be valuating them base on their current circumstances. AFAIK, Xiaomi does indeed do their own tuning for the TV, the panel is definitely not manufacturered by them, probably similar arrangement with Philips. More imporant is these companies probably have their own in-house QA rather than the likes of Sterra which basically just change logo.
Again, doesn't matter if TCL took over Samsung or what, they are indeed a panel manufacturer as of now, and a very decent one with tech probably a generation or even less behind Samsung in some aspect. Whether they are good with their QC is another thing all together, but they are definitely OEM by classification.