I won’t comment on the Proton thing as the other replies already explained it well.

IMAP is weak security since it uses basic authentication, so I’m not sure why you think integrated support for IMAP makes a service better or more secure.

IMAP does not make it more secure, it makes it so that you can use any email client and aren’t tied in to their proprietary apps, like you are with Proton. You either need their desktop app or their bridge.

IMAP having weak security is not an issue (also I’m not aware that it has, you can use imap with plenty of authentication options), because all of your emails are encrypted if you set it up correctly. Someone could steal my every single transaction with mailbox.org and not be able to do anything with the data.