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).