If you argue that "any right in any patent claim of Facebook is invalid or unenforceable" you lose the patent grant. So you can indeed argue the (patent) suit

If you argue that "any right in any patent claim of Facebook is invalid or unenforceable" you lose the patent grant. So you can indeed argue the (patent) suit is invalid or unenforceable, and you'll probably win, but by doing so you'll lose that right in the future: you'd need to immediately stop using react. In effect, the license is revocable by facebook, without cause. In short; it's not a patent grant; it's merely an agreement not to sue over past infringements.