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.
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.
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...