Honestly when I play against players who hit very flat I try to take them out of their comfort zone by playing a lot of slice backhands and just try to play deep and consistent with my forehand. Playing slice backhands to them is the most logical approach to their game because it is basically psychics you are dealing with here. You aren't going to be able to surpass psychics and hit low floating balls flat and think that they are going to go over the net and drop like topspin does. The only way they can counter this is either take pace off so gravity can take into effect earlier or actually play with topspin and both situations are winners for you.
You must be very careful against psychics because they know where you are going to hit the ball. I think he means it's the physics you should focus on.
By the way, being a flat hitter myself, I can tell you that the guys I really hate playing are the hard hitting topspinners. It's simply impossible for me to consistently hit the ball as hard as they do and keep it in since I don't have the spin working to keep the ball in the court, plus the topspin they have gives them more consistency than me. So against good topspinner I end up feeling like they can hit it harder than me and keep it in more. I'd much rather play a flat hitter.