Propjet tended to be a term used by manufacturers in the US when the first turbo props were introduced. The term propjet was used long before the experimental propfan emerged.
Rigpiggy is probably correct in that it stems from the marketing departments.
The Electra was commonly referred to as a propjet and this aircraft was not succesful in the passenger market because it was an advanced (for its era) turboprop that was introduced as turbojets were becoming popular.