he not a steel type but he can use steelium Z anyway, this is normal
Crobat can learn Steel Wing through TM and as an Egg Move. So, of course it'll be able to utilize the Steelium Z.
The way Z-Moves work is by using a move the Pokemon has. So this Crobat for instance will most likely know Steel Wing, a Steel Type move, so will use the Steel Type Z-Crystal (Steelium Z) to unleash Corkscrew Crash, the Steel Type Z-Move.