That is the worst case if the process had leaked that part of the heap, but it is an optimal case on process exit. On OS with any kind of process isolation walking over most of the heap before exiting as to "correctly free it" is pure waste of the CPU cycles and in worst case even IO bandwidth (when it causes parts of the heap to be paged in).
Pages can be completely avoided to be paged in if the intention is to just zero them. The kernel could either just "forget" them, or use copy-on-write with a properly zeroed out page as a base.
The point is that you do not want to do any kind of heap cleanup before exit. The intention isn't to zero the pages, but to outright discard all of them (which is going to be done by the kernel anyway).
