What is milk product in cocoa butter?
Any time you see something like "may contain" after an ingredient list, it's a warning that it might contain trace amounts of that substance and so could be harmful to anyone who's allergic to it. Basically what they're saying is that they didn't intentionally add any milk or milk products to the cocoa butter, but since they make other products at the same factory that use milk they can't guarantee that some hasn't slipped in somehow.

So assuming that cocoa butter is the only ingredient then it would be pure cocoa butter, at least by the legal definition.