As I said in my comment, because he makes two references to the private SO offering known as Teams, and none to the Jobs monetization. [1] Therefore, it isn't something that the current CEO thinks the next CEO should focus on, therefore, it can't be what is paying the bills. Whether it works for a developer or not I can't say, I have no personal experience using any job board. Just I don't think it can matter much to the bottom line of the company based on what Joel wrote here.

[1]: "especially Teams, where we are starting to close many huge deals and becoming a hyper-growth enterprise software company very quickly" "And we want to make it possible for knowledge workers everywhere to use Stack Overflow to get answers to the proprietary questions that are specific to their organizations and teams."