I've seen that definition here before - who came up with "prog" and "progressive" being two different labels? Where and when was it established? By who?
It's not an establishment, but it's more the way the words are used.
Prog is derived from "progressive," which was a description of how the music was written. Music that belongs to prog the genre, however, is not, nowadays, always progressive the adjective, so the definitions have naturally diverged ever so slightly.
There are lots of sound-alikes in the genre, who aren't so keen to tread new ground as to pay tribute to the old stuff - not a criticism, Transatlantic are prog-not-progressive and they're widely considered to be brilliant.