IANAL either but I think the part about modifications by third parties means that you don't get a license to an additional Facebook patent you might infringe by

IANAL either but I think the part about modifications by third parties means that you don't get a license to an additional Facebook patent you might infringe by modifying it. So if you change React to add some functionality that infringes on a patent Facebook holds you don't automatically get a license to that patent.