No, the Weight on Wheel sensors are definitely on the Main landing gear as well. There are two sensors per leg dedicated to this task; the failure of one, or a very tail-heavy condition during loading with e. g. the rear cargo hold filled to the limit and nothing else on board (extending the NLG shock strut too far), will trigger the "WT ON WHEELS" caution light in the flight deck. This requires maintenance attention before the next flight and is not releasable under MEL.
So I am still surprised that the WOW sensors allowed a gear retraction at such a premature stage.