I don't know about Rob, but nylon rod would definitely be sturdier--have fun carving a slot through it though. Also, since (common) nylon rod has a 1/8" smaller OD than CPVC, there's very little room for error. CPVC has been used in the past and I haven't heard of it failing on anybody, and it's much easier to makeYou never showed what your plunger rod ended up looking like, is it just a piece of CPVC with a huge slot in it for the priming bar? Wouldn't Nylon rob but much more sturdy?
tl;dr- nylon is stronger but it's not worth it. He likely used CPVC.