"All revolvers are pistols but not all pistols are revolvers," is a phrase I have heard for decades and has been in print longer. A revolver is a subset of pistol as are single shots, pepper boxes, fixed multi-barrel pistols, semi-automatics, bolt action, et al. I have seen revolvers called pistols in copies of ads long before Borchard and Mauser came up with their designs in the late 1880s. I recently reread Billy Dixon's auto-biography where he calls whatever revolver he is carrying a "pistol". I'm pretty sure even Sam Colt's 1836 revolver was initially called a "revolving pistol" in early descriptions if not patent request too.
I don't know when the movement to separate the revolver from the pistol category began but I suspect it was not long after semi-auto pistols became feasible devices. If one wishes to use form rather than function as a description I have no problem with it unless one's undies get in a bunch and they try to "correct" everyone. There are much more important definitions to clarify such as calling pistols handgun. and vice versa.
