As I was intrigued by this discussion I did a short internet research concerning ivory imitations and it turns out there are resins that imitate ivory quite well, including the Schreger lines.
Here is more information from the GIA website, for reference:
https://www.gia.edu/gems-gemology/fa...-with-a-pseudo
PS: However, upon more careful examination of the photos from the GIA website one can notice that the cris-cross pattern of the resin samples is less regular and without the optical interference effect of the genuine Schreger lines. Also, it is quite apparent that the difference in coloration is also visible sideways, along the length of samples (see for example the tusk-shaped sample).
So, I still am pretty sure the hilt in question is made of genuine ivory. Polishing out the black spots (or polishing the interior of the hole) may reveal the characteristic "dentist" smell resulted from burning the ivory.