The funny thing to me is, knowing Joel, the interns and developers being interviewed where the few, the proud, who could solve fizzbuzz. Nowadays that's... quaint given the prevalence of leetcode style interiews.
Having passed a Fog Creek internship interview a few years later, I can say the questions were definitely harder than fizzbuzz. I don't remember the exact questions (One was a technical design question about how to do some sort of optimal screen layout, which ended up being an NP-hard problem). Very similar to the questions asked by Microsoft and Google and many other companies around the same time -- whiteboard coding in C to do some sort of data structure manipulation, etc.