In a DM, if a user isn’t already part of the DM, the poster does get a popup notification that the user won’t be notified unless they are invited. We mention other users in DMs all the time, but don’t necessarily want to invite them to the DM.
The way I see it is that Chat could be set up differently - to automatically notify & invite a mentioned user to the Chat. But wouldn’t that be counter-intuitive if someone simply mentions them during a discussion and the person is then granted access to the chat? A catch-22? Or would it be simpler just to use @ username if an invite is not warranted?