Schnizel willingly made an impossible move to state that he was ending the game. It was a test, to see how Zero would react. If Zero had just let it slide, he would have had Suzaku delivered to him right then and there.
Instead, Zero ignored it and moved his pieces so that he could continue to play against Schnizel, ignoring a victory that was placed right in front of him because it was not won by himself but instead was a product of Schnizel, and Zero wanted to beat him to prove something.
Schnizel walked away with a better idea of how Zero would act in the future. It was a win-win for him. Either way he would gain information on Zero. Further, Zero didn't buy it (which he didn't) he could continue the game and possibly beat him and make him reveal his identity. If Zero took the bait, he'd just lose some Eleven who his dad had made a knight and he hadn't liked very much anyway.