One thing that never seemed to sink in for the folks at Ancient Aliens is that not everything our ancestors drew was meant to be literal.
Now granted, I'm not sure how they concluded that alien reapers with plague scythes were responsible for the Black Death, as I can't sit through a single episode, but given the big deal they make out of some pantheons consisting of humanoid gods (protip: there are plenty of reasons why ancient peoples would have those sorts of gods that CAN'T be written off as "aliens"), I wouldn't be surprised if they looked at some plague art without realizing that the reapers depicted therein were ABSTRACT.
I do not like this show, my point being. If you're gonna devote your airtime to alien conspiracy theories, do some actual research.
...ironically, Rock is flipping out at an alien, in a fan-continuity where aliens are a well-known fact of life. He just doesn't like theories that boil down to "aliens did all the work for us pathetic hairless apes".