I'd say the line is that, if you can independently deploy just a single part of a repository, that part should probably be in a separate repo. In the article,

I'd say the line is that, if you can independently deploy just a single part of a repository, that part should probably be in a separate repo. In the article, the author stated that "X decided to merge and deploy just the back-end changes". To me, that suggests that perhaps the backend is an independent component entirely decoupled from the front end. At the very least I think you should avoid making changes to multiple independently deployable parts in the same commit.