Given the quote "I don't have time to review any outside code contributions or GitHub pull requests. Feel free to fork the code." I'd hazard a guess he's fine w

Given the quote "I don't have time to review any outside code contributions or GitHub pull requests. Feel free to fork the code." I'd hazard a guess he's fine with people building atop his work, just doesn't want the time-sink of reviewing + merging those contributions into his tree and maintaining them. As he states others have indeed done so and built research projects atop it, it would seem to be worth keeping open source.