[Standards] Re: LAST CALL: XEP-0424 (Message Retraction)

Gotcha, though in either case, it's the same client that {retracts|edits} the message as has sent it (and presumably chosen the id), so if it cannot figure out a sensible strategy for generating ids, it presumably can't

Precisely. When validating sender, the risk of id clashes stops beieg an issue since the sender knows they need unique IDs for corrections. So using message@id instead of origin-id or stanza-id is the way as in the correction XEP.