SpoilerIt is heavily implied, as said before, that Ishgardian blood is needed to properly awaken "the inner dragon" so to speak. The fact that hyur can do it as well means that there would have to be a small amount of crossbreeding. Not to mention that it's also treated as less like a biological issue and more of an outright curse set on Ishgardians for eating Ratatoskr's eyes, in the process of integrating into Ishgard, the hyur could've also been stricken by this curse, or there might have even been hyur in Ishgard whenever the eyes were eaten in the first place.