But why? What's the USP here?

Maybe there's a demand for a more feature-rich mail client on iOS, perhaps syncing accounts and filters from the desktop app. But this doesn't seem to be that, at least not at this point.

It's impossible (other than mutual TLS) to do mfa on the native ios mail app without trusting a third party for one of your factors. Hopefully Thunderbird will support it...

They have a paid email service now. This is likely for that and for other users who want a thunderbird-based, free e-mail client.