The big news it that they removed the PATENTS.md file that made it nearly impossible to use for anyone concerned with protecting their IP. Good on them for making right with the open source community.
Due to the way implied patent licenses work, if Facebook distributes React without explicit restrictions or conditions, they're automatically granting a license to any patents that are inherently infringed by React itself. This is a pretty well-established area of US patent law, see e.g. https://www.wilmerhale.com/pages/publicationsandnewsdetail.a...
if fb actually wanted to prevent someone from using react, they could just revoke the copyright grant.