Think about this. There's really no full range of motion for the targeted muscle in most
exercises. The quadricep does not extend fully even if you go real deep. The other joints
and muscles involved prevent that. A leg extension where you can stretch the muscle far
back in at the starting position would come closest. Plus you would have high tension at the contracted position
which you don't get leg pressing or squatting. Same with the bench press, just becuase you lower the bar to the chest
doesn't mean the pecs are fully stretched, a fly movement would be better for that. Or think about deadlifts, who decided on the diameter of 45lb plates decided "full range of motion"? The fact of the matter is you obviously don't need "full" range of motion to grow and most of the supposed best movements for growth are partials.
Now, loading the muscle at the stretched position is probably good for a growth stimulus, but again, not necessary.
If Kevin's legs aren't growing it's not because he doesn't go deep enough. Partial movements were sufficient in the past, as On_Swole said.