No implication, no “abuse” of MX records: your mail sysadmin is straight-up-and-down doing it wrong. MX records are “mail exchange” records. Specifying the outgoing relay MTA in an MUA (which Discourse is) is not something that MX records are involved in. I’m not aware of any MUA which performs MX record lookups on the name configured as the outgoing relay MTA. Even stub MTAs (which are a curious hybrid) like ssmtp don’t do MX record lookups on relay names.
As for references, there’s a good chance that the backfire effect is going to bite you, but if anyone will be convincing, it should be Google, with their answer to the question, “What is an MX record?”:
Each MX record points to an email server that’s configured to process mail for that domain.
So, an MX record for outgoing.example.com says, "the MTA you can contact by talking to the name in this MX record will accept mail addressed to <anyone>@outgoing.example.com`. It does not say anything about that MTA’s willingness to accept, forward, fold, spindle, or mutilate mail for any other domain.