You are correct. Brake fluid gets water and other stuff in it over time. That makes it conductive. The problem was the hot side of the switch was always live because it uses the same 12V source as the brake light switch. Which has to work when the car is off.

https://www.zehllaw.com/ford-cruise-control-recalls/

My experience with electrolytic corrosion is a track develops and it gets more and more conductive over time.