To what extent can Gielis' so-called "Superformula" be used to improve the efficiency of WIFI antennas?
Well, there's absolutely no information as to how the improved antenna would be improved, or even what it would look like exactly, which makes it hard to say. It's generally hard to analyse complex antenna designs; maybe they have something there.

For the 3D design case, it is generally true that expressing something as a generated surface of a function is much more compact than decomposing it into polygons or pixels. So provided they have a good algorithm for inferring the curve that describes a particular shape, that part is very plausible... if you're making things that are that kind of shape. However, I don't think the file size is much of an issue in 3D printing.