Not true. Not sure who would say it, but it's not accurate. And there were multiple PX levels, PX10, PX10L, PX10LE, there were some models with N as well, IIRC, etc. Maybe that's why someone would spread inaccurate info, confusing features between models? Dunno.

Best way to date is on components - if the derailleur is still intact, look for date codes on the back of the derailleurs. Or maybe if hubs are original, look for date codes there.