Still, there's been some confusion about just what he means when he says "social interaction," which is something I hope I can clear up.
First, yes, all roleplaying games are fundamentally about social interaction to some degree or another. When we say "social interaction" we're talking about in-game behavior. In most games, the main in-game activity is combat or investigation or something else adventure-y. We're talking about a game where the equivalent to a boss battle is a big fancy party -- or, heck, even seeing a character my character has a significant relationship with for the first time after being separated for a while.
Second, it doesn't mean "social combat." When I sit down with a cleric and a Rakshasa to talk about boys, I'm not trying to convince either of them of anything. I'm trying to figure out what the cleric wants out of a relationship (because the fundamental issue at hand is "which boy?") and find out more about the boys from the Rakshasa (because she's known most of them longer than we have). Almost all the conversations we have in that game are like that. We're trying to solve a mutual problem, uncover an issue, or work all the angles on an idea, rather than convince someone of something or another.
Or, sometimes, just chatting. Here's an example, very slightly edited from the original log to make it more readable:
I lean out over the parapets. "Nice night."
"Gorgeous night," agrees Reswet."A blue night," says Seban, joining the two of you. "Sometimes the Sea of Fire burns hotter than at others, and it turns blue or purple.""When do you find these things out?" I ask. "I swear, you already know more about this place than I do."He chuckles. "We actually talked about it at dinner. I've never seen the Sea of Fire. I hope to soon, but the others have said we hardly go to that part of the city."I nod. "Mostly, what, the docks, and the industrial parts of the city? Azer, fire giants, and so on?"Reswet nods. "I've been to the Merchants' District once, but only briefly, and never anywhere close enough to see the Sea."
Yes, I think this is fun. I spend 4-6 hours a week on this stuff. I'm a freak. I kind of consider the solo game to be practice for real life socialization, that's how crazy I am.
Third, this is a style of game that doesn't always work so well at the table, and the very immersive, hyper-focused variation on the style that Trollsmyth and I have developed pretty much only functions in text, as I think the above example makes fairly clear. There are a lot of little things about text that help support this style of game. It's easier to stay serious and focused for long periods of time, for one, though the biggest is probably that it's much easier to separate player and character. If you can't see how roleplaying in text could be more fun than doing it in person, you're not likely to have any interest in the kind of game we're describing.
There are a lot of games that could be described as focused on "social interaction" that would handle social mechanics fairly well, or even require such critters. For instance, the phrase might describe a game that was mostly about politics -- talking to people to get them to do things. There's no reason it couldn't apply to a game that was played at the table with more than one player. If you've got more than one player, you've got varying levels of interest in all the activities of the game, including talking to people, and at the table you're going to have different comfort levels with "talking in character" and otherwise exercising your social mojo.
In such a game, I'd probably want my players to have some social mechanics at their disposal. Probably not "you roll and the king agrees," but certainly something like D&D's reaction mechanics, reputation mechanics, perhaps rules to allow players to gather "dirt" on their foes with dice rolls, that sort of thing. In that kind of game, the game would be "what do we want, who can give it to us, and how do we push or pull them into doing what we want?" Playing out how that happens wouldn't be crucial.
In the solo game, though, the point is the relationships and how they're affected, so the little details of how everything plays out are important. Little jokes between lovers, accidental allusions to secrets my character is keeping, that sort of thing. Most mechanics we could use would be, at best, unnecessary, and at worst would replace the stuff we find most fun. Unfortunately, the terminology we have available doesn't really make distinctions that fine, and these are all types of games that are unusual enough that there aren't clear categories for them. Even "a game focused on social interaction" is unusual in itself, never mind all the variation within what those words could mean.