Seems like it kind of depends on where you grew up and what your experience is.
Having done much hiking in the Pacific Northwest, I am familiar, with bears cougars, and snakes to some extent.
However although it probably should have, it would not have occurred to me, that gators would be present, on a Disney owned hotel property. As I have no experience with gators, and know nothing about them. Remember these folks were from Nebraska. I doubt they have much knowledge as well. Especially if the lake was small and looked man made, by Disney.. I would assume no dangerous critters. Big difference between that and wild area. Where I would assume gators, snakes etc
I would have thought the no swimming sign was due to submerged logs, or weeds, or no lifeguard.
And in my mind, wading in 1 ft of water would not be swimming, it would be wading.
You go hiking in a park with snakes ( or even rest stops on the freeways), they have signs warning you not to go off the trail and that snakes are present. Seems like a gator sign in this case would have been a good idea. For all us rube tourists