Is the issue the lack of real-time updates? In principle, you could work around that using a separate WebRTC channel for live updates, with the slower Dropbox sync serving as the source-of-truth. (It does indeed take >=5 seconds for Dropbox to sync a collaborative app in the way the author describes, in my experience.)
I think concurrent editing is where Dropbox collaboration falls down.
E.g: you edit the intro paragraph, and your buddy adds a conclusion, both while offline (or simultaneously). Dropbox can't/doesn't resolve the conflict in your files, and instead picks a winner.
This is still solvable by your app, but requires additional logic/storage above what the "shared folder" model provides.
Is the issue the lack of real-time updates? In principle, you could work around that using a separate WebRTC channel for live updates, with the slower Dropbox sync serving as the source-of-truth. (It does indeed take >=5 seconds for Dropbox to sync a collaborative app in the way the author describes, in my experience.)