I started using tuta until I realised they don't support IMAP. Something to do with not guaranteeing encryption (which isn't even enabled by default) but has the convenient effect of locking you into their apps
Tuta is always encrypted I don't know where you got the impression that it was optional or that they could somehow magically make it work over IMAP without a bridge like proton.
IMAP can do TLS though (IMAPS).
Did they say what's stopping them from using that (and requiring the encryption!)?