[FrankenPC]

(I have no idea what I'm doing...) how much does 1 cu-foot of water weigh compared to 1 cu-foot of limestone? water is about 63Lbs / cu-foot. limestone is 163 Lb / cu-foot. So, 1cuf stone in water is 163-63=100Lb (it's already partially buoyant because it's IN water). Then you need to evacuate water using air bags that are ABOUT 1.5 times the blocks size. So, the whole payload would be 1 unit block with 1.5 units of air. or 2.5 units total. Much bigger than the video suggested. But not unreasonable.

[bradleyland]

It's much easier if you work in metric units (as usual).

The density of water is 1,000 kg/m³.

The density limestone is around 2,500 kg/m³ (it can actually go down to around 2,100 kb/m³).

This means that you'd need to displace more than 2 to 2.5 times the volume of water in order to float a limestone block. Keep in mind that volume goes up (roughly) as a cube of the increase in linear dimension, so the difference won't be quite as dramatic as it first seems, but that's still a lot of flotation required. Certainly much larger than the floats shown in the video.

The animal-skin bladder theory is particularly problematic, because the volumetric efficiency of spheroid animal-skin bladder floats isn't particularly good. Water would fill in the space between bladders, requiring much larger float assemblies than if they were able to construct larger, single-chamber bladders. The video depicts float assemblies that aren't even as large as the blocks they were transporting. I found that rather disappointing for an engineering-driven theory.

[FrankenPC]

Yeah. So, the "hats" of bladders on the block can't be too high because the depth of water would be impossible to maintain. Therefore, they would have to be wide. That would limit the number of floating blocks you could have at any given time. It would also present a problem with the gravity conveyor to the top of the pyramid.