He doesn't have eyes so all he can do is fixate his divine sense on them and scan them without restraint. He wanted to get to the bottom of their strength. It is pretty rude to scan another demon king so intensely but he doesn't care. He is already in danger. A little more danger can't hurt. But he failed. The two of them successfully defended against his probe. That only reinforced his suspicion.
"Interesting."
He finds it interesting that there are two demon kings ahead of him who are apparently strong enough to defend themselves. What's more interesting is that he found two more demon kings whom he couldn't ascertain if they were using an avatar or not. These four demon kings appear to be in their main bodies like him. And like him, no one is sure if they are avatars without souls.
