Yeah, but those people in the video didn't get pushed "really hard". That was the whole point. They were reacting violently to pretty small pushes (albeit ones that Phillips was fairly grounded and they weren't).
But the point is to assume
that the force pushing them back is larger!
If you don't assume that Phillips is right then you can't test any of his situations. He claims that by relaxing you can project the attacker's force back to them, so when he pushes he claims that his arm strength is not the major factor. We have to assume that he's right if we are to analyze any of the scenarios.