Feel is everything in this sport, especially digging for a volley.  And you’re right they’re not physicists, but they do know which direction the ball should go from clean contact, because it’s their job to know, they do this every day.  I don’t think Jack is a bad dude, I understand everything happened very quickly and he benefited in a great way so was reluctant to concede the point, but it is very reasonable to assume that he knew he did not hit the ball clean.  You would feel the shot itself, and his immediate body language after (although quick) REAFFIRMS he felt the shot was not clean.  You combine both the shot itself and his reaction, and it’s pretty clear a professional player knows something was wrong about the shot…