System Does Not Matter makes sense from a particular perspective. I don't believe it is really an argument that system does not literally matter though. There is a particular vein of role playing game design, popularized by Vampire - The Masquerade that defines role playing games so strictly that there is really only one system that all role playing games fall under. It's pretty much a thin veneer over freeform roleplaying. The basic conceit is fairly simple. The GM has a story in mind before play even begins. During play the GM presents a situation. Players declare actions for their characters. The GM decides what the fallout of those actions will be with an eye towards his plot. The key element here is that players are supposed to pretend that the presented game rules matter. There will be dice involved sometimes, but that is simply to maintain the illusion that players decisions and what's on the character sheet matters. It does not. It might be taken into consideration, but that is ultimately up to the GM. A convenient short hand for the way these games operate is Stat + Skill = Whatever. The Whatever meaning a number the GM pulls from his butt, and can arbitrarily change if it suits his or her purpose before or after the roll.
The key selling points of Stat + Skill = Whatever games is not the game, but rather copious amounts of setting material that fans can pore over. Actual Play is focused on trying to puzzle out what the GM wants players to do, providing color or characterization, and applying elaborate setting knowledge. Don't get me wrong - these roleplaying games are still games. They just happen to fundamentally be the same game. It's a play style that is pretty close to a tabletop version of Zork.
Other examples of this philosophy of role playing game design include Numenera, Shadowrun, Legend of the 5 Rings, and AD&D 2e - especially towards the end of its life cycle. Planescape, Dark Sun, and Ravenloft were pretty much TSR aping White Wolf. 5e seems to push this way sometimes, but there is some respect for OSR related play styles.