Yes, they are that dumb.
MLB should be thinking of ways to close the ability to steal signs rather than do what all sports leagues do, which is make an example out of one team (when many other teams also do it, or would if they could.) Rulebooks are generally pretty dumb...they were written before advanced technology, and now many of the “please promise you won’t do this...” rules in a multi-billion dollar business are pretty absurd. I’m sure there are plenty of high tech ways for pitchers and catchers to communicate without hand signals that are visible to anyone in centerfield and on every tv camera as well.