It's probably fairer to say that Joel solved the chicken-egg problem by leveraging his existing community and fanning antipathy against experts-exchange. If the model was primarily SEO I'd expect them to have an easier time bootstrapping their non-tech related communities, especially now that they're investment-backed and can put money into user acquisition:

http://gaming.stackexchange.com http://cooking.stackexchange.com

All that said, I appreciate the thoughtful reply. It's rare for people to link to supporting data like this. And maybe you're right! Upvoted.