Which one has more value? A file included in the repo, written in legalese, on which Every single source code file's copyright headers point to (and the README file as well), or an ambigous blog post?

Reading the file in its current form (https://github.com/facebook/react/blob/b8ba8c83f318b84e42933...), You lose the license to make, have made, use, sell, offer to sell, import, and otherwise transfer the [React] Software.