It doesn't work that way. If you can take any given hour and trade it for a certain amount of money, then you're lucky. I don't know anybody who can take 12 hours and trade them for $240, or say $600 (as you are hinting that this time is worth significantly more than $20/h).

I could get a second job, but that would pay less per hour, would start at a higher (fixed) number of hours, and bring a lot of responsibilities. Plus, it would cut into my precious free time.

Only for hyper-demanded successful freelancers time is completely fungable and arbitrarily tradeable into dollars. The rest of us can't monetarize their free time so easily. And I think a $250 saving for a day's work is pretty good for regular people. Besides, I think there was a bit of hyperbole in the "10 hours"...