Kyran did not make a move when the three magical beasts started their attack because he already sensed the old man at the border. Given the situation, he did not think the old man would keep his hand while his people were slaughtered.
Showing up like that would end their facade, but that was the least of their worries at the current flow of things.
Besides, Kyran only suggested the old man leave the scene in the pretence of getting abducted because he wanted to give the Eidums time to breathe from the Nobles' harassment.
Whether the old man returned to the scene at a later date did not assure the Nobles would stop their harassment, especially after the news Dusk provided.
