Poly-B pipes are not the same as PEX pipes. PEX is short for “cross-linked polyethylene,” which is a different type of plastic than polybutylene.
PEX is manufactured using a different method, and the result is a plastic pipe that’s much better for home plumbing. PEX can stand up to heat, pressure, and trace chemicals much more effectively than Poly-B. PEX is common in modern homes, whereas the use of Poly-B has been entirely discontinued.