True, but “force” as used in that paper is so vague I gave up. Force is applied somewhere, and it makes very little sense to add up forces in different places. Although if you ignore GR and add up forces everywhere, you get F=0 (Newton’s third law), which is well below the supposed maximum. And trying to decide whether the net internal force of a system (0) is an extensive property is a bit silly :)

Admittedly, power has much the the same problem. Saying that P <= P_max is some sort of physical law requires a lot more explanation of what it would mean than the paper even tries to give it.