Yes, George says so on his DVD.
While I get his point I don't really agree. Have a look here - others disagree, too:
Double bass help?
(Posts #11 and following)
My reasoning as for _why_ getting the left foot up is super simple.
Do a max test with each foot separately. What speed does your right foot do? What speed does your left foot do? See a difference? If that difference is but a tiny one - congratulations. If not - you have work to do. Because overall your foot speed (ok, speed isn't the focus on this thread, but you get the idea of being balanced, don't you?) will be twice the speed that your weaker foot can handle, not twice the speed that your strong foot can do.
Just try it for yourself: Focus on your left foot for a while, play it in isolation, plus practice both feet but lead with your left foot (up to higher tempos). You should notice some difference in your control/confidence throughoug the speed ranges.
BTW, George's new DVD has some killer exercises for double bass/foot control.