I saw abyss at 0:25
I guess miners have to be careful
Also you can fly under map if there are several such holes in map and you have elytra and fireworks.
don't suppose those might've been skylands, do you?
I wouldn't be surprised if it's set up as there being the main overworld, but go high enough above the build limit, and you can reach a separate sky-island world (from which the way back to the main overworld is by going down, perhaps just far enough below the clouds in the skylands that there's decent building space, but high enough above the void level that the horizon still looks like normal sky when you fall back into the overworld, which in a super flat world, basically means elevation 1 or 2 would work).
Closer inspection of the "abyss" shot shows that the "abyss" stretches across both horizontal ends of the screen during the entire shot. The pond is dammed as if to stop the water completely flowing down into what ever is below, though there is a bit of a spout sticking out with a 1x1 waterfall flowing down, suggesting that it may serve the purpose of being a way up.
I can't exactly tell if the sharp drop in the cliff is because the land there is so close to void level or not, as the world could have been made in a super flat world, and the shaders, resource pack(s), and other visual settings don't show any recognizable clouds beyond the sky box textures, but I still wouldn't be surprised if the "abyss" isn't even an abyss at all, as it could instead be a divide between two neighboring sky islands.