Practicle, no personal. He can learn about them all day ling, but that doesn't means he has any first hand experience. That being said, he's hunted down orks, eldar, and heretics most noticibly, with a small excursion against a tyrnanid force and brief run in with sylth, rakghoul, and strixis- but depending on the year this story takes place- Tau are not common knowledge. Senior members of the Deathwatch for sure, but the idea is Dominous is a recruit.