Some apps are written using canvases instead of hildon windows. When creating plain X.org windows you can set them to fullscreen, too. I think it's also possible with qt or gtk/hildon, but I'm not 100% sure as I've never needed that and that's just my common sense. Anyway, it can be set in source code before compilation, it's programmer decision.

You can try what sixwheeledbeast suggested maybe. Modified hildon-desktop can be a way to do it also