It does not; insertable is purely based on whether or not you can create a new resource within said resource.
When you're performing an action, you're not really performing a POST on the resource itself, but rather the action within the resource. The fact that an action exists implicitly means you can perform a POST on the action.
Thank you, you've confirmed what I thought should be true. I asked because the spec left me wondering about it. Something like the phrase you used here ("whether or not you can create a new resource within said resource") would be a good clarification for the spec wording.