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).