This basically answers it.
From a privacy standpoint it’s genius, especially when you use a privaterelay email address. Only the developer you signed up with can send emails to it so even if they sold your email address, the third party advertiser can’t reach you.
The result is that apps and services begrudgingly offer SIWA in their app when they offer the other option because of the rules.
Their displeasure in this is so bad, that nowadays some services don’t offer a SIWA button for you to login on their website out of spite despite offering you to sign up with it in their app (where it’s mandatory). Leaving users with the choice of either only using the service through the app or by logging into the app, change their email and setup a regular password via password reset.