That's exactly my impression too. Authentication is hard. And this isn't some random site wanting to store user data, they're doing invoice management! (So... not quite handling money on behalf of users, but pretty darn close in terms of liability.)

Regardless of your feelings on Big Tech and Privacy and whatnot, this absolutely looks like a security downgrade to me. If I were someone looking for para-financial services like this to phish with fake users for fraud purposes, I'd probably start with a site like Slimvoice.

Personally I think there's a good argument to be made about the benefits and tradeoffs to allowing giant cloud companies to control the idea of "identity" on the internet. But if there's any market segment where big companies with deep pockets and extensive technical resources bring value, it's this one.