Pronghorn evolution : a matter of fact?

The voiceover tells us, matter-of-factly, that the Pronghorn evolved to outrun the now extinct American Cheetah [Yellowstone - BBC2]! How do they know? Surely this is hypothesis rather than fact; isn't it possible that the Cheetah failed to evolve fast enough to be able to catch the "fastest antelope in the world" or to vary its dietary requirement to devour other fauna in the region? Presumably, long before it became extinct, this species of Cheetah had occasionally found something to feed on!