If people quit NCSA on Friday and start coding Navigator on Monday, you could call it "from scratch" or you could call it inevitable disclosure. (I don't know the exact timeline.) NCSA had nothing to lose so I don't really blame them for trying every tactic they could to make money from Mosaic once the Web became the next big thing.

For better or worse, the university IP lawyers considered it related enough to sue. (No endorsement implied!)