It's ridiculous to think that walking on an incline will reduce the roundness of one's backside. I don't know what those girls looked like before. Perhaps the extra fat they carried back there was the only shape they had and with the weight loss, so went their a$$es.
In order to get that lovely shape back there, you really need to concentrate on developing your glutes, and it's true that just walking an incline won't cut it, unless you are purposefully working those muscles back there. Just doing squats and lunges without good form and focusing on the right muscles won't do very much for you either.
I like to use Bret Contreras' Strong Curves for my glutes work. Definitely seeing a difference.