It wouldn’t be centralized. There would be multiple instances and you’d choose which one to use to host your account.
I like your idea, though. It doesn’t solve the problem of moving your account from one Lemmy instance to another, for example, but it does reduce barriers to entry for other fediverse platforms.
I don’t really know the details of how “log in with your Google/Facebook account” works on other sites, but I imagine it’d work the same way. I’m no expert on this stuff tho.