Ah, the "one click" thing was for exporting from Tuta rather than importing, my mistake.

Either way, I found this:

We’re thrilled to be able to offer email import (in beta) currently to Legend and Unlimited users. In the future, email import will become available for all new paid plans but as we have just released it in test mode, it’s temporarily necessary to limit the number of users importing.

— Posted on 2025-02-12

What worries me a little bit is the "will become available for all [NEW] paid plans" part, as if old paid accounts won't be getting this new feature. But I hope that's just a phrasing mistake/interpretation error.

I'm currently on Legend because they had some deal a while ago where you could get on that plan for the same price as Revolutionary, but I don't see import in the mobile app. I could check the desktop app later!

Edit: yeah, when I'm using the desktop app, I see the import option, and clicking it does not prompt me to upgrade my subscription. (It just shows me the screen to import email as you'd expect.) Again, this is on Legendary for the time being. We'll see what happens in March when I have to downgrade to Revolutionary, if clicking Import prompts me to upgrade. I hope not.