Yeah I thought this was a weird take too. Too often people take privacy for "I can do what I like". IMO deleting something you've sent to someone else is not a privacy concern at all.
I don't agree with it myself, but there are people who seem to want to frame "the right to be forgotten" as a privacy issue.
IIRC it is possible to have some clever encryption so that the person you sent your message to can prove to their own satisfaction that it came from you, but they cannot prove to anyone else that it came from you. Which gives you plausible deniability; you can always claim that your contact forged the message.
Can't remember what the algorithm is called.