(Having watched the video): No, GP does mean "old-world cities with immigrants". However, GP meant to end with "that never developed in the new world".

The story, in brief: old-world cities, which were crowded, always had a fresh supply of (non-immune) immigrants, and had lots of human/animal contact, allowed the development of particularly lethal diseases. Such diseases did not develop in the new world, which lacked such cities. Thus there was no "americapox" to spread from the new world to the old.

I repeat the story without knowing/endorsing accuracy.

or even better, let's just say "American cities" if that's what we mean, no sense using dated and confusing terminology.

no -- OP meant the opposite of that. "Old-world" meaning anyone that traded with or interacted with the descendants of the first cities in the fertile crescent (aka anyone from Africa / Asia or Europe).

"old-world cities with immigrants and animals in close contact allowed diseases to develop that never developed in the new world"

Except that the cities in question are not exclusively American...