UN did not claim this. Adrian Zenz who works for US interests claimed it meets UN definitions to justify XPCC sanctions despite knowing full well this was going on for past 5 years when policy started. Just like how this entire narrative was started by Zenz in a presentation at a US funded panel affiliated with UN, but not UN itself. The only real UN thermometer is US getting 23 countries to support XJ as a human rights issue, and China getting 56 countries to support it as re-education. UN will never rule this as cultural genocide (what it is) let alone genocide (what it isn't).
CCP also raised 1B out of poverty and is responsible for 30% of global growth, so really on balance a few million Uyghurs+Tibetians+HKers+ eventually Taiwan getting their human rights violated isn't convincing arithmetic for billions in the rest of the developing world who relies on Chinese growth, when magnitudes more are being screwed in Middle East for no tangible gain. CCP bad, but also good.