See here, with language from #33. The language looks like it's drifting towards the in-flight opencontainers/runtime-spec#395. But however that PR goes down, the container process is gone by the time the post-stop hooks run, because that process existing is the stop event. I'd suggest echoing the spec:
The post-stop hooks are called after the container process is stopped.
although I'd be a bit happier if the spec used “exits” instead of “is stopped” (to avoid confusion with SIGSTOP).