That's a decent way to train what we call the magnetic line of force in Hermetics.
A farmer's walk works well too as does holding a bar at the top of a deadlift.
His method would be superior for getting the energy to the fingers versus the hands, which is something most people have problems with. Not a problem if you just want to punch someone. A big issue if you want dexterity with your tendon strength.
I will add the best method I've seen for tendon strengthening, that is commonly available, is Olympic weightlifting. Nobody has more tendon strength than those guys.