The spec requirement is just that the itemprop is used in at least one item. There's no suggestion that I can see that being referenced by an itemref does not constitute a usage, nor that the itemprop is copied, and that it is the copy which is used not the original. So I think the spec requirements are fulfilled, and the microdata is valid.
Moreover, consider the intent of the spec. The underlying idea is that an itemprop that is not used by any item is pointless - a waste of typing - an therefore must be an authoring mistake. That is sufficient for mark-up to be considered invalid. Since that is not the case here, there is no justification for making such a construct invalid.