> For instance, "move this hunk two commits up" is a task that makes many git users sweat.

Citation needed. You split the commit anyway you like, e.g. with the mouse or using cursor movements or by duplicating and deleting lines. Then you move it with the mouse or cursor or whatever and squash it into the other commit. Maybe some people never intend to do it, but then these probably also don't want to learn JJ. I guess this is more of a selection bias, that these that care about history editing are also more likely to learn another VCS on their own.