What does it mean to move a single flight control to its full deflection?
Yes, that is correct. Moving any control surface until you hit the stops is considered "full deflection". For the purpose of maneuvering speed, this is the speed at which any single control surface can be fully deflected without risking damage to the control surface or structure.

Pitch is full back/forward on the yoke or stick, which is where the elevator stops movement. This isn't always 90 degrees for ailerons, especially in aircraft that have stick controls versus yokes.