Huh. I've owned a few manual-transmission cars over the years and they all disallowed this trick -- pressing the clutch would disengage cruise control just like a tap on the brakes.
Yeah, pressing the clutch will do that. But you can pop the car out of gear without pressing the clutch. (IIUC) the synchros provide some positive holding force that holds the transmission in gear, but you can overcome it. Also that force goes down with the amount of torque being transferred through the transmission, so you can make it easier by playing with the gas pedal a bit.