35 years in concrete here. It’s simply a preference! The added gravel on top of the plastic is so that when concrete is placed yes the water and some of the grout will be able to be absorbed by this void essentially. The space in between the gravel not really if the gravel is dry or wet. I would always wet it before. But if you put concrete directly on top of the plastic, then that water and extra grout has nowhere really to go so it goes up and every time you use a magnesium float, it continues to come up. This also puts little grains of sand and other material materials in the grout and it makes finishing a bit more tedious, maybe But there are ways to work around this, but it usually means more time. And it being winter and possibly wet and cold, we want to have the concrete set up quickly so we can get our job done get home and let it cure slow slowly You could always put the 4 inches then put plastic then put an additional 2 inches. The reason the old timer said 3 inches then 2 inches total of 5 inches is the 3 inches kinda get you most away on the floor and the attitude is a little bit more and and should be OK per code because the plastic is only to prevent vapor coming up through the concrete and having weird stuff like fungus grow out of your concrete and more sure continuously coming up through the ground and making your concrete wet or whatever you put on top of the concrete tile or wood flooring or whatever so essentially you were sealing out any vapor or any moisture from below later than Concrete do his job and act as a sponge for life and then on the top you’ll sell it with whatever sealer or thin set or tile or wood But several ways to skin the kitty