That referenced extension is for collaborative writing of active notebooks in real-time.
Everyone can edit everything when you use that. From here:
“There cannot be any conflict, everyone works in sync on the same document.”
It sounds like what you seek though is more along the lines of nbgitpuller, litegitpuller, gitautopush (see discussion about it here, & more recently here and here), notebook{sharing}space, nbsanity, or nbviewer.