But if you need to split the commit in hunks, and add a commentary per hunk, isn't that just a sign that you really should just split your commits?

That's the most killer feature of git, that it's so easy to slice your commits any way you desire, and then redo again. The use case of taking a chunk and commit separately is so common it even got a special mode in the add command. That, and the super fast jumping between branches is what set it apart from contemporary version control systems.

The extra context provided by the review tool is gone when the review is done anyway. Review systems come and go, but the commit log is for eternity.