This is the intended behavior as the cookbook is written currently. The cookbook needs to stop Chef Client after the upgrade to ensure that cached data such as the location of ohai plugins on disk is refreshed.
Currently the only way for a recipe to make chef exit that works for all cases is to raise a fatal error; the reboot resource has some functionality to end a Chef Client run early without a fatal error, but it won’t force Chef Client to exit in all cases, so the cookbook uses a fatal error instead.