Well, that's bullshit. I've known a lot of geeks, and while it's far from universal, I've known as many of them to be stoners as any of that other stuff. Probably more, in fact.
See also: I'm a sports nut for Magic the Gathering
Personally, I have weed to thank for at least one of my primary geek obsessions. Back in the early '90s, when Magic: The Gathering was still a new thing, the people who introduced me to the game were inveterate, degenerate stoners. We're talking comically stereotypical stoners, here. I mean, these dudes would wake and bake pretty much every day, had zero ambition (beyond staying high all of the time) and could spout entire Cheech and Chong routines from memory with the slightest provocation. They were also massive geeks. Role playing games, anime, all of that shit. At the time I was a very casual stoner, but I spent a lot of time with them because they were high school buddies and, well, fellow geeks. It kind of annoyed me that they were always stoned, but what can you do?
Anyway, one day they all started playing this game. Every time I went over, it was all Magic, all the time. And I mean every.fucking.time. It looked kind of janky, with its cartoony wizards and lightning bolts and them saying shit like "Specter attacks, take two and discard, dude" between bong hits, so I resisted playing for a long time. Months, I think. Eventually, though, I found myself indulging in a few bong hits myself -- rare, but not unheard of for me, then -- and suddenly I found the game much, much more interesting. That is one of the effects of weed, after all -- the most mundane shit can suddenly seem fascinating, once it knocks the part of your brain responsible for disdain on its ass. So they taught me to play, and that was it. I was hooked. I didn't even need to be stoned the next time, because it turns out that not only is Magic a great game, it's also a fucking complicated one that's a hell of a lot easier to play sober than high.
On a similar and strangely symmetrical note, years later it was playing Magic that got me back into smoking weed. I'd been not smoking for six months or so when I met some new people to play Magic with. I invited them over, they pulled out some weed and I was like, "It's a been a while, but sure..." A few months later, one of them hooked me up with a nickel bag (after I explained what that meant to him; apparently the slang has changed...) and ever since I've returned to the ranks of regular smokers.
That's not the only thing, either. Video games are so much better when you're stoned it's not even funny. Science fiction movies offer a lot more mind-blowing ability when your mind is already half-blown with bong rips. Bad zombie movies are about 500 percent more bearable when baked out of your skull and Mystery Science Theater 3000? I am 99 percent certain that shit would not exist without the magic of marijuana, a belief bolstered by the number of pot jokes in the early episodes.
When you think about it, it's not that surprising. Back in the '70s, when having a sweet airbrushed painting on your van was the cool stoner thing to do, what did people display? Dragons, wizards and wizards riding dragons. That's some geeky shit, no matter what decade you live in. With weed being legal hereabouts, I expect you'll see a lot of geeks coming out of the closet in regards to their smoking habits. We probably won't paint wizards riding dragons on our vans (er, Priuses), but now that the stigma of prohibition is gone, you can probably expect a lot more of us to admit that we pulled a couple of bong rips before we went to that midnight showing of Evil Dead. Maybe people will even realize the reason we have that distracted, faraway look has less to do with being socially awkward and more to do with being stoned out of our gourds. Probably not, though. They'll be too stoned themselves to notice.