...now, I was going to say that you can't play, say, Traveler with D&D rules, but there /are/ RPGs with which you could fairly accurately play other RPGs, by not only modeling all the same characters, challenges, and setting info, but also while aping the other game's system artifacts...
...it's a tad hilarious to do, actually.
My name appears in the credits for d20 Traveller rules... T20 Traveller's Handbook.
It's got some quirks, like it was playtested with half the XP rate given... so high-level play is not as well balanced as it should be.
The game engine can be thought of as a system architecture, yes... but it's much more profound an impact than playing, say, Lego Star Wars on Playstation2 vs Game Cube vs Wii vs X-box.
Still, lots of people talk about, "Lets go play some X-Box" without stating which software they're going to be playing.