Because of how the UN works, a statement from the UN that something isn't a genocide isn't necessarily evidence that it's not a genocide under the definition the UN gives. Obviously, there's the massive elephant in the room of not upsetting member states in a way that might make them less likely to engage (especially when they're as capable of ignoring the UN as China due to their power and having a permanent veto on the Security Council), but there's also the fact that the UN's got lots of subcommittees and working groups that regularly put out statements that contradict each other. A few weeks ago, news was going around that the UN had released a report saying there was no evidence of a genocide against Palestinians, and it was true that they had, but it was from a body whose job was to represent Israel, and it contradicted hundreds of other reports saying there was overwhelming evidence of a genocide.

The Office of the High Commissioner on Human Rights is a pretty big deal, though, so if they say whether or not something's a genocide, it's worth paying attention. However, the report you linked doesn't make a statement one way or the other - in fact it doesn't contain the word genocide even once. It does list an awful lot of things that the Genocide Convention includes in its definition, though. A report that presents a lot of evidence of genocide but little against, and says crimes against humanity are happening while stopping short of making a definitive claim of genocide isn't something that says a there's no genocide. The people who've shown you that report and said it says there's no genocide are misrepresenting it to push an agenda. Several countries recognised a genocide based on the evidence the report gave (although obviously they weren't all without an agenda of their own).

The tankie talking point is that nothing bad is happening to the Uyghurs, which is blatantly false.

Bad things have absolutely happened to them. They suffered violence from Salafi-Jihad terrorists[1][2] and suffered mass unemployment from the West’s (led by the US, ofc) embargoes on Xinjiang cotton, the region’s main export, which only adds tinder to terrorism potentially re-igniting. Given the last 25 years of the US showing the world that it doesn’t give a rat’s about Muslims, and given that it considers China to be its greatest adversary/enemy, one shouldn’t uncritically accept its professed motive: to protect poor, oppressed Uyghurs.