Interesting, though not 100% convincing for me. EE is riddled with ads and yet clearly see the need to charge as well. SE and SO both benefit from the ability to target ads really well but programmers in particular are notorious for not clicking which will offset that for the core three sites at least.
In terms of the careers and CV model, it works fine for SO / SF but the rest of SE contributes nothing in that area which begs the question how do they support themselves (just low key ads?) and if they don't then aren't they just an overhead? Or are they deemed to basically be a very cheap by-product of the existing SO technology? In which case they don't really need to bring in much, just enough to cover hosting and ad sales so maybe that's the answer.
But I'm still thinking that there has to be something more.
That said the point about the home workers basically being cheaper is well made - there are two ways to be profitable - high revenues or low costs.
I'm assuming that there is one, I don't believe Joel and Jeff are stupid, it's just not clear to me what it is.