The fonts are made with freshly written extensions to the RoboFont editor; notably, the original source drawings are made in quadratic bezier splines, not the typical cubic ones.

Metapolator (a project I co-founded) uses Hobby splines and has not been used so far to develop Variable Fonts AFAIK.

I filed an issue for your nice suggestion, https://github.com/TypeNetwork/fb-Decovar/issues/6