I think Schneizel knew this move is invalid. It was all part of his plan.
Lelouch's weakness is that he's only a genius within the calculable, definable, strategic environment of a battle. Sure, he does make mind boggling surprise attacks, but they all make use of the environment somehow. When something completely illogical happens, he becomes useless.
This time, Schneizel used an illegal move to see how Zero reacts to something unreasonable. Lelouch was so confused by this that, instead of just declaring vistory, he moved his king back, in an attempt to make the situation reasonable again.
Lelouch has a very fucked up mind, and now Schneizel knows of this.