There is no guarantee in the game, in or out of character, that a hero style should be usable by another type of creature. Almost all martial arts are so usable, admittedly, but:
-You could use a charm-that-functions-as-knack setup or make knack-based extensions that compromise on the issue and allow others to practice the style without being able to achieve the same level of mastery. Personally, I wouldn't bother with that path, because...
-Lunars are magnificently powerful killing machines, meant to be terrible in their fury and power, and they get no unique capabilities as far as combat goes. Martial combat Knacks are potentially quite powerful, extremely in keeping for the Lunar concept, and give Lunars something they can call their own.
And I take it they aren't talking?