It depends on your definition of what an RPG is. For me, it's playing the role of a character and having a non-linear story.
See, here's something that's been bothering me. People are really hating on linearity in games, like Madoc on the Kickstarter page,
We dislike linear story, we hate linear story, no, we truly despise linear story. If you asked us what is as pleasant as being poked in the eye, we'd have to answer "linear story".
Sure, I enjoy more open-ended nature in games, and choice and freedom, however linearity isn't the spawn of the devil, it isn't like getting poked in the eye. Having a linear design to your game allows you to control for the player in the environment, and when you can do that, it allows you to weave a much tighter, specific experience that plays out in a finely crafted path. Whereas in a open ended game, you slap monsters about, if it's linear, and you know the players path, you can specifically craft the challenges in the environment, you can design each level to precisely how you want it, giving you a richer, more fine-tuned and thoughtful experience. People celebrate the advent of the sandbox, but in reality, it's the illusion of freedom at the cost of quality of content- sure, you can go around killing whoever doing what you like, (though many sandbox games really don't provide many options besides explore or kill), but in a linear game there's a clear plot that you adhere to, and by doing so it's a much tighter narrative because the action follows it closely. Basically, what I want to say is that RPGs can be linear and still be great. Look at a game like Deus Ex- even though it gave you some options, it was for the most part a linear game with a specific plot where you travelled from level to level, giving you the choice of how to approach each objective, but ultimately still having those linear objectives. A lot of really good games have linearity, it's not a bad thing, we don't need to hate it. Used well, it provides tight, tailor-made experiences that are every bit of rich as open world experiences. In fact, you could argue that it allows for so much more intense experiences, because the game can be made specifically for that character, for a specific arc that plays out, you can polish it to a perfect sheen. Comparatively, an open-world game must be vague, because the character is always different, it has to be approachable from multiple angles, so it can't be tailor made to the same degree.
I'm just playing devil's advocate here, because even though Sui Generis is flipping conventions on their heads and innovating in so many respects, it doesn't necessarily mean that traditional design doesn't have merit as well.