Technically all Garage Kits are "customs" as well once assembled and painted. Many of the limited edition 12" figures being sold in monster circles these days use resin heads (I can't remember... is it the Executive Replicas? or one of the other companies stocked at Amok Time?). I don't think they will discern.
For them... it's the fact that a Jack Pierce figure was actually produced, and exists in limited quantities beyond what the demand will be past the initial social media publicity. Being a show exclusive gives it some extra cachet as well.