This is likely a case of in modding circles. In many hacking forums, "PSP" has become an anachronistic prefix—a relic from the mid-2000s when "ISO" files (disc images) were king. Back then, "PSP ISO club" was a real term used by piracy groups sharing ROMs.
Explained: PSP ISO Vs Eboot Files & How To Install/Play Them pspisoclub gta 5