Provides neither trustworthy attribution stats nor reliable privacy
Advertisers would have to be born yesterday to rely on any data from this Apple scheme. It relies entirely on trusting anything that contacts their website and claims an attribution to be honest when it says that it's a copy of Safari that's seen a conversion from ad campaign X on website Y. If the privacy protections work as intended there's no way for them to verify this by tying the reports to actual orders or visits. Any ad scammer could just set up bots pretending to be copies of Safari reporting successful conversions from ad campaigns on their fake sites and the advertisers would have no way of distinguishing them from the real thing.
Worse still, I don't think it will reliably give the level of privacy claimed, because in many cases advertisers will be able to use other information such as IP addresses to match customer information with ad attributions anyway. This won't stop fraudsters since they can target scenarios where this doesn't work. It feels like this would fail to protect actual user's privacy whilst simultaneously protecting scammers faking advertising views and conversions.