The edit window is always just a thing that clients do, see e.g. https://xmpp.org/extensions/xep-0308.html
The whole point is that it's not guaranteed to show the edit everywhere, and that clients can choose what they think is the most reasonable course of action.
It just means that the mentioned scam is not a good argument, since this is not happening on other protocols that support edits currently.
Sorry, I see what you mean now. The relay would have to reject edit messages that are too late, because the client could be offline and doesn't get a timestamp from the relay, only from the sender. But people don't want smarter relays, so that's out of the question I guess.
Delayed sending on client-side is definitely better than not trying to solve it at all!
