That’s not what it is saying. During the Dinosaur era, mammals could not evolve larger because the dinosaurs were eating them. Only small rodent like creatures could survive the onslaught of the dinosaurs.
Once the dinosaurs were extinct thanks to that large asteroid, mammals began thriving, and gradually became larger and larger and apes became a reality.
So how did the asteroid know to only kill the dinosaurs but not the mammals.
Pretty fishy man...Oh wait...what about fish? How’d they survive?