Because the listing that was leaked said "Soundwave PR". Plenty of people recognised that as "package refresh", particularly those who are involved with the retail world (and really, after the last couple of years with quite a lot of re-packed toys, it should be a well-known thing by now). Plenty of other people, though, latched onto it as "Prime", and no matter how many times it was explained that it was "package refresh" people just clung to the Prime Soundwave thing. I expect, even after the official Hasbro announcement and reveal of the toy in question, to see people asking in a few months "where's Prime Soundwave? It was in the leaks...".

Also, that listing that Griffin links to may be the only one left that hasn't been fixed to remove "Prime"