Sure. The point of the top knee is to disrupt the other guy's base with outwards pressure. Especially combined with using your hands to control his neck/arms. Ig you are doing what your should be doing on the bottom, the top guy can only hit with one arm, and it is just an arm punch with no weight behind it. Add to that is the fact that as soon as he commits to trying to punch he is no longer able to stay in a locked position to defend against the sweep. The best time to sweep a guy is almost always when he is trying to hit or access a weapon. That is when he is most vulnerable.