Wouldn't "React Native cross-platform bridge core moves to OSS" be a better title?
What was it in before?
Based on the commit comments, it sounds like it has always been C++, just that this is the first time it has been exported from FB's Mercurial mega-repository to the open source GitHub project.
I'm not familiar enough with React Native to understand what this commit is really doing and yes, this HN post doesn't seem particularly informative either.