The older strips show Dilbert as a socially inept idiot. In this one the joke is that Dilbert is bad at making jokes.
A common stereotype is that when couples are double-dating, the women will go to the toilet together to gossip about their respective partners. Men don’t do this. Dilbert’s joke tries (and utterly fails) to play on this.
So it’s a joke about engineers’ inability to make jokes.
“Oh, now I know where I’ve seen that cat before”