I don't think I read any where that Abraham was making the fonts "proprietary" .. I do believe he said "Some fonts are becoming commercial". And proprietary doesn't necessarily mean closed source. Although in most cases it does, because proprietary software if modified holds ownership "usually" with the owner. As for commercial software it can be licensed many ways. It's main goal is sometimes revenue. As I suspect it will be with Abraham.
Commercial software can still be free software. GNU is not against making money.. it's about free software
the question is how Abraham intends to make it commercial.