Regardless as the maintainer of that GitHub clarified in a closed pull request, it’s not actually allowed on Github to have a license that blocks the ability to do forks and modify the programs yourself, I never knew this but it says it on the page he linked.

basically it seems if you post a project as public on Github, you implicitly grant a license to fork and use the code regardless of what it’s terms say since you need to follow those terms for the Github platform usage. The section 6 I’m not sure about though, cause the terminology confuses me, I can’t tell if it means that it can be supercedes or that it supercedes a private license

it seems his intent isn’t to dissuade people contributing, he’s just been burned a few times with GPL violations so he’s changing the terms to prevent that