Indeed, the PR hadn’t been merged in yet, some review happened but no merge yet. Always review anything yourself and not just blindly use something so you can avoid unnecessary build headaches.
Sage 9 also runs a script on top of it that adds framework-specific (which can be selected) modifications.
See this discussion that also shows how to create a Sage project using the update branch:
You create a new Sage 9 project using that PR, then you overwrite all the files (except vendor/ and node_modules/ of your own project with the ones of the newly created Sage 9 (PR) project).
Then you can diff/merge everything, which means that you keep your own modifications but keep the new ones from the PR. Most modifications happened in the webpack-related configuration files (“glue”-code), so it shouldn’t be that much to adjust.