re: The science behind the lakes hypothethis
I'm not up on the details here, but I would think that there would be a lot of other ways to use radar to confirm "liquid" than simply the smoothness of the surface.
If you use common sense, though, the only way to get a large, very smooth geological surface in ANY environment is to involve a liquid at some point (even salt flats), so that even if these are frozen ponds now, they were lakes at least once.