This isn't true and it seems you have never used the app, so you don't know if an IMAP implementation doesn't actually have an edge over it.
IIRC, Mailbox actually had the ability to read your emails - some of the features they provided meant their servers had to have access to your inbox. Now, for this to work on IMAP, that meant giving Mailbox your email and password, and for Mailbox to store that in more-or-less plaintext.
Now as an enduser using Gmail, would you prefer that Mailbox stored your password somewhere in us-east-1, or an easily revokable oauth token?
