The whole 'what makes an rpg?' thing is a tough one. I think its a question of how the game plays; if there is a lot of exploring involved then a game 'feels' more like an rpg than it's core gameplay might suggest. Take a game like Super Metroid on the SNES. It's not exactly benchmark rpg, you could call it a platform shooter, but the nature of the explore the map and collect new items thing makes the overall gameplay experience more like a rpg.
I've not played dark arms, but from the reviews I've read and the screenshots I've seen, it doesn't look a million miles away from a game like Zelda, just with guns. You know, top-down map-based monster splatting. The point is that it has more depth to it than a simple shoot-em-up.