They changed the storage layer and the query planner in some parts to work correctly with their new storage layer. So it‘s a fork and not behaving exactly identical. I have seen some different execution plans for the same model and data because of the implications pf the changed storage layer and query optimizer.

AWS is basically never sharing any details, only when it‘s benefical in terms of marketing. The changes they made had not been possible with an extension in the past, and they are most probably still not fully possible without forking. But thats not problem, they are free to fork and merge upstream changes.

For MySQL any of the big users like facebook & co. Are running (heavy) forked mysql versions with changes they need.