Ryu really was ignorant to the fact Isemeine actions had put a target on his back. He had been too far away to sense their killing intent and he hadn't deployed his Spiritual Sense.
In truth, though Ryu was a bit cautious of the Astral Winds, the true reason he didn't release his Spiritual Sense was deeper than that.
For one, it was greatly taboo in the cultivation world to scan others with your Spiritual Sense. Even if he had used it to see, the moment he swept through the wrong cultivator it would cause an unnecessary clash. No one would care that he was blind, that was just the way of the martial world.
Secondly, releasing his Spiritual Sense would reveal his inherent resistance to Chaos Qi, something that would bring him endless trouble.
