As in the oppinions above, the Hippopotamus ivory could get there via different trading routes.
However, it is equally possible that the hilts are made of diferent kinds of ivory and were simply wrongly labeled.
For example since there is such a big negative image associated to elephant ivory, some traders may feel they make their goods more saleable by labeling them as Hippo ivory. Or, since marine ivory tends to command lower prices, they may mislabel their goods as Hippo ivory in the hope they can get a better price.
Just guesswork and speculation...
