Yes, voluntary hierarchy is unanarchistic. Classic example is the Nazi Party in Germany, who designed a very effective and efficient machine of screaming madness that slaughtered anarchists and innocents alike. They didn't force the anarchists to obey their hierarchy. They just killed them. Even if every Nazi was wholeheartedly in support of their leader and their obedience, and most of them were, it's still an existential threat to anyone who's not part of the cult.

But even on a purely abstract basis, voluntary hierarchy is unanarchic, because anarchy means "not hierarchy," and unanarchy means "not not hierarchy." Hierarchies can absolutely exist in anarchy, whether they literally gun down anarchists or not, but they are not anarchic. The more hierarchy, the less anarchy, of that specific group. Anarchists could be educating women next door, and they wouldn't cease to exist just because they were in the other room adjacent to the "obey your betters" club.