I'm not upset at being excluded from a job at Discourse -- in fact, I'd probably exclude them from my search! Quite the opposite, my time is valuable to me.
Just because I can't take a week to work for you on a contract basis doesn't mean I lack motivation or ability. It just means I have priorities that don't include sitting in front of a computer screen 24/7. I may have a wife and kids, or I may be an Olympic runner, or I may be struggling to make ends meet with other contract work, or I may be taking care of a sick relative. Who knows?
All I'm really saying is that there's a happy medium here. Some way for you to give me a programming assignment, and for me to complete it in a reasonable amount of time. Offering to pay me just means that you want a LOT of my time -- time that I likely don't have to spare.
If I were a junior dev and desperate for work, then yeah, I'd make this a priority. But Jeff isn't talking about junior devs -- he's talking about the best. And the best already have jobs and don't need to prove anything to anyone.*
* (I'm not saying I'm one of the best*)