It's okay, Skyfire- Tsukishima is making the same sound that he'll also deny later. Pressing close to the mane, he has no chance to see how the skeleton moves, and can only trust his traveling companion to take care of him.

That means he has no way to see how the skeleton blocks the blow with its blade, rolling along with the force of Skyfire's strike. Getting back up to its feet, it starts to circle him, blade held out in a trained stance. It's considerably different than dealing with the bats.