The creators can say it is a turnip if they want, it means bugger all. And you simply cxontradict yourself there, saying that the 'only' thing you need to do is play a role, but then going on to say that is not enough in Half-Life. At least try and be consistent.
In what way is Diablo more of an RP than a shooting game? All you do in Diablo is click on bad guys until they are dead and pick up what they leave behind. Your 'advancment' merely gives you more attack modes- just like picking up guns in Half-Life. Why does slashing = RP and shooting = arcade? It's dim nonsense.
In Diablo, you do not control the plot, you cannot converse with people (instead jjust hearing what they say), you cannot affect the game world in any way more significantly than you could in Gauntlet, R-Type, Final Fight, or any darn game where the objective is simply to destroy as many bad guys as possible.
What is this failure of logic that equates the term 'role-playing' with 'game in which you can become more powerful'? That is NOT what the darn phrase means!
It is not enough to just be in a role- that happens in nearly all games. Nor is your personal development anything to do with it, as that happens in lots of games that are nothing like RPs. At the very most, that can be called 'an RPing element', simply because it occurs in many RPs. But association does not mean identification. Lots of FPS games have machine guns in them; this does not mean that only games with Machine guns in them can be FPS games.
An RP game is one where your adoption of that role is one that has an affect on the world around you, rather than being pre-set. Computer RPs are nowhere near in scope with this compared to tabletop games, but they carve out a niche for themselves. BG counts, KOTOR counts, I have serious questions with FF counting but it is on the line. Diablo is way past that line, with the arcade shooter.
A term like 'RP' has to have meaning. If you just make up a random meaning that has nothing to do with what the words mean, you simply do language a disservice an ecnourage great confusion.