I had my DD 15 weeks ago and it has quite unexpectedly happened to me.
Not that I am complaining, obviously. I am a size 10/12 though so nowhere near as stick-thin as the DofC.
I put on 2 stone while pregnant and had a reasonable-sized bump. I have eaten whatever I liked since the birth and have made no effort whatsoever to lose any weight or tone my stomach (other than EBFing if you buy into that theory).
Maybe she has been working out but I would say for the most part it's down to genetics and bloody good luck.
If it makes anyone feel any better, I may have my pre-pregnancy body back but I also have a baby who doesn't like to sleep at night so you really can't have everything.