Curiosity: He was actually referring to the popular net-tzitzis. He said that the point is the fact that it’s made specifically to reveal as much of the skin as possible, so it can’t be categorized as a beged for hilchos tume’as eilim, nor for hilchos tzitzis.
Rav Belsky doesn’t address the issue of polyester, but I am guessing that he would follow Rav Moshe, although my guess is as good/bad as anyone else’s. (He usually paskens like Rav Moshe Feinstein.)