Squating is not the only way to call upon the highest anabolic hormonal responces during training. Can apply a higher rep scheme to the DL, cleans, etc with equal results. A higher quality test & GH release, among other chemicals functions in the body, give the opportunity for muscle mass growth and strength advancements. As far as the squat & push-press goes, that has to be right up there with a select few other superior growing movements. Even in a program without any squating at all, but with exercises like the C&P, DL's, cleans, etc, a subject could expect to make alarming gains. For the sake of experiment, try a set of 12 to 18 reps of the clean & push-press, starting each rep from the floor. Around the 6th or 8th rep, the important of this exercise should become clean as a whole body and gaining exercise. And very much coupled to the affects that higher rep squats have on the body.
As you stated about the squat,I can witness to the fact that I gained 30lbs in 6 weeks with doing the breathing squats, pullovers,bench & rows as my only exercises in that period. I'm sure also I could have gained muscle mass on the upper part of my body if I had only done the squat it's self, as you suggested. I was 16 at the time and an easy gainer, but never the less, just about anyone could gain pure muscle to some degree. I work with athletes and some have to slap on 10 to 20 lbs in a rather short period of time. The do a squats (heavy breathing style), chest, shoulders, back & abs exercises also. Good Luck.