Short circuit - bypass or impede

Cognitive dissonance - conflict from holding two conflicting beliefs simultaneously

So it's a way to either prevent or stop mental frustration due to conflict between two beliefs.

For example, X needs to be fixed, but fixing it will take too much effort. "It is what it is," so we'll just accept it being broken as a given and work around it. If you didn't short-circuit the line of thinking, you'd spend a ton of time on something that can't reasonably be fixed.

Believe A is true. And believe B is true. Also believe A and B cannot both be true at the same time. That is cognitive dissonance.

In this case it is "short circuited" aka bypassed by saying something dismissive. Doing so suggests there is nothing that can be done about the circumstances behind the dissonant ideas, so it's not worth thinking about.