I've talked to some of their people there. It's true they only hire from line guys. Expect to spend 6-12 months doing that before moving to the right seat. After that, another year or two until you make it to the left one. I've talked to a couple of people that fly 737-800s for Miami Air that spent a few years at National and they can't speak enough good about it.
I almost went there but really didn't think I should be doing line service when I have 1400 hours. Maybe I'll regret it someday, who knows?