@avlcharlie borax being a fine powder would prevent them from crossing it. But the reason they use borax is because if ants eat it they can no longer absorb nutrients and die. If you get the queen to eat it then you kill the colony. Ants share food so this is an effective tactic.

Problem is borax alone wont be seen as a food to ants so other than preventing them from passing by being a powder it wont actually kill them. Ant poison is usually some sort of food mixed with the borax, specifically sugar. This causes them to eat it and share it and kill the colony.

They probably recommend borax simply because it is assumed to be an effective poison on its own due to those reasons even though it is not. It must have sugar mixed with it to work.

Interesting fact though, insects dont have lungs or a respiratory system like humans do. They breath through holes in their skin and direct diffusion of gases. It is entirely passive and the whole reason why all insects are small. They simply cant get much bigger than they are without suffocating.