By chance, do you happen to know if the Mindstorms NXT (the old one, before EV3) software was based on the same toolkit? I always wondered what UI framework it used, it had an unusual look.

It was not... it actually was, IIRC, a LabVIEW program with some tweaks here and there. The UI was basically a LabVIEW VI front panel with a LabVIEW 2D Picture Control. Most of the program logic and the compiler to the NXT was LabVIEW G code.