In Context of a VPN Provider like Mullvad a ULA is required. One of the rare cases where ipv6 nat is a thing.

NAT might be a thing in Mullvad's case, but there's no link between that and needing ULA.

Explain because as far as i can tell, to make it impossible for i.e. google to tell me apart from another mullvad user, me and that other user must share the same IP adress.

You don't need to use ULA addresses to use NAT. You can use any GUA prefix too (preferably one you own or are assigned in some way, rather than squatting on one, to avoid the risk of clashing with someone else).

Well it's more private. Also there's only one proper IPv6 implementation (using GUAs) and most people.cant get one

It's exactly as private as doing the same service with a GUA prefix. Just get a PI block and pick a /64 from it to use instead of whatever ULA prefix they picked at the moment.