@atbeaune How's that different from signing up on any instance?
The reason I singled out Google and Microsoft is because a lot of people have identities established with them already (e.g. email addresses and OAuth), so if they wanted, they could also slide into the business of providing people's Fediverse identities, too.
Imagine if you could simply assume you could reach someone's Fediverse presence by adding an @ to the beginning of their email account.