Single unit pullbacks, medivac shuffling, and focus firing, all together, are commonly seen in pro-level matches. The guys are superhumanly insane. The current pro players can sustain 500 APM during battles. The only reason they don't do this even more is that they are usually fighting on two or three fronts at the same time, while managing base and unit production to the second.

The real limit of the players is the single viewport and the single instruction input pipeline. Managing dozens of units with those heavy restrictions is pure madness. That's the main difference between RTSs and MOBAs. MOBAs are defined by strategy and tactics, teams win and lose because of those two factors (at pro levels). A game like Starcraft II adds another factor: players can simply break down when they're not able to keep up with the game. Even at pro level, players can be overwhelmed by the amount of information they have to process in tenths of a second, over and over again, for 15-25 minutes, or they can get into a spot where there's too much to do and not enough time to do it.

AI has two advantages. It has better information channels, in the form of perfect vision, while humans have a single pair of eyes that have to scan through the screen. It also doesn't get tired over the course of the game.