On the contrary. Greg's work is creating a new market for expertise. If you want to sell your time in 20 minute intervals, it's quite difficult to do so today.

I'm not convinced anyone really WANTS to sell their time in such small intervals.

A counter example: He mentions graphing data as one option. I'm pretty sure people with excellent GNU plot skills can make a GNU plot script that does what I want in 10-20 mins, and I really can't be arsed to figure it out myself everytime. Seems like the type of expertise that is currently hard to sell and might benefit from this sort of site.

At least, in this case, the jobs won't be, for example, creating a yelp clone for $250.+

Sooner or later, all freelance coder sites drift towards cloning Facebook for $250 or less.