does the standard mandate any of that?
The backends are either cloud controlled (hence associated with your subscription) or device controlled (associated with the serial number, purchase, drm... ignore your fancy key, everyone will use the TPM in their laptop/phone). And yes, All protocols have similar restrictions to hide information from one side and another (publisher, relying party, etc) but none are better than the isolation you get today with same-origin. And those were put in place by the same advertisers, so it will not be a problem either.