He's not the only one to take issue with Diablo, or even Final Fantasy games, as RPGs.
So, since I have given this subject *way* too much thought over the years, I thought I'd give my take on the subject.
The reason most folks want to exclude Diablo is because of its lack of "role-playing". Diablo is hack and slash, its all about combat, combat and more combat. Along with some character progression and then more combat.
The thing is, I think Diablo has exactly the same amount of role playing as Mass Effect, which is to say, none.
Role Playing is social
In my opinion, role-playing can only take place with other human beings.
While Mass Effect has a ton of choices in it, and Diablo has almost none, except "will I explore the left corridor first", I don't think selecting choices off a dialog wheel is role-playing. If you have color coded responses, with the teal choice being good, and the red choice being evil, are you role-playing?
If you can go online and find out exactly what you need to say to bed that sexy blue chick, and if you can bed her every time by making exactly the same dialog choices, is that role-playing?
If you don't think this happens, go look at the many, many faqs for Bioware's amazing Baldur's Gate II, that will tell you just what you need to do to bed the good girl *and* the bad girl in the same game.
Is this role-playing? I say no.
Role-playing is inherently social and if that were your criteria, you'd have to exclude all single-player RPGs, while including things like LARPs and games with a heavy online component, like MMOs and (wait for it) Diablo.
RPGs are mechanical
Now the other way to define an RPG is mechanical. Think back to the days when Gygax and Arneson were slowly transforming the historical wargames they played into the first RPG. What were they adding to the games they played?
These games were already social, so it wasn't that aspect.
No, what was added was personalization and persistence.
Instead of controlling an entire army, moving your French grenadiers here and your cannons there, RPGs put you in the role of a single hero. And not only a single hero, but a hero you got to name and who was different from other heroes thanks to his ability scores.
But what really made RPGs different from what had come before was persistence. If I played Napoleon, no matter how badly I trashed the British last week, this week we started the battle over from scratch.
If my Hero reaches second level though, at the start of the next game he's still 2nd level and the game has changed along with him. I'm fighting Hobgoblins now, not Kobolds and my character continues to grow and change, and the game along with him.
In short, mechanically speaking, RPGs feature customizable, persistent avatars controlled by a human player.
One last point
Bringing this discussion back to the original question that started us off, another consideration with Diablo specifically is the game's heavy online component, where multiple human players link up online, form parties and take down monsters together.
Diablo and Diablo II are proto-MMOs and so, even if you reject my reason for putting Diablo in the same category as Mass Effect based on mechanics, I think the heavy online component, which is played with all the devotion of WoW, puts Diablo firmly in the RPG side socially as well.