Kind of. We already had a native UI tree running in native (the same way the browser has it's own internal representation of the DOM). The difference in this release is that we rewrote it in C++ and made it immutable. That means instead of having a different UI tree in each platform (one for iOS, one for Android, etc), we have one C++ tree that all platforms use. And since it's immutable, it's thread safe and we can read layout and commit it from different threads if needed.

Reconciliation is still done in React on the JS thread, similar to React DOM.