But forking duplicates the process space and has to assume that a write might happen, so it has to defensively reserve enough for the new process if overcommit is off.

COW is not shared memory, it’s an optimised copy. There is no way to guarantee that the optimisation will hold forever thus it is a form of overcommit (and indeed the reason most unices overcommit in the first place): most children will not touch most of the virtual memory they inherited but any can, so if you require precise memory accounting you have to account for that in the same way you account for large anonymous maps.