I thought PEX was just an industry term for HDPE pipes, but no, you’re right. The X stands for crosslinked and irradiation is one of (the more expensive) ways of achieving it. It sounds like most common PEX is made by adding peroxides to the HDPE or LDPE before extrusion and the chemical causes the thermoplastic to become a thermoset after the heat and pressure of the extruder. Neat stuff.