It's not flat; it's gliding. Some songs make a swing dancer want to minimize the dip / maximize the glide. Some songs make a swing dancer want to increase the dip. It's still swing, and Lindy Hop is what they're doing. The majority of what I have seen from Lindy Hop dancers is minimizing bounce. Bounciness is nowhere near the top criteria for validating Lindy Hop or anything "swing".