I think you being a too literal in your interpretation of the phrase. 'the best money can buy' is figurative - it means 'the best available, regardless of circumstances'. The 'money' part is figurative; it represents resources.

I think it's a little unfair on Joel Spolsky's team to suggest they might not be good at what they do based on a semantic argument around whether 'the best money can buy' can be interpreted as using resources or just cold hard cash. Considering the things they've built they're very evidently a highly accomplished group of developers. I don't see any need to argue about this; they're obviously using what they believe are the best tools for the job. You can tell that from looking at their results. Whether they're using free, open source things or they're buying tools isn't relevant, and certainly isn't a sign that "your development team is probably really bad". They're clearly not bad. They built Trello and StackOverflow.