Yeah, CDCK switched to all-mail-receiver, all-the-time, around 7 years ago (based on the initial commit to the mail-receiver repo; I don’t recall whether we’d started using it internally before publishing it).
Given that Discourse just wants to get a copy of the email and never think about it again, IMAP (and, I presume, JMAP) are massive overkill. Appeals to “more standard” are swamped by “sucks less”, and hoo boy, does running Postfix in a container suck so very, very, very much less than configuring POP3 (or IMAP) mailboxes both at a mail provider and in Discourse.