It would be nice if someone answered Kevin McMurphy rather than down voting him, after all it was a constructive question and from the point of view of a non native speaker, this sentence does seem like it should not be in the instrumental case. Any explanations as to why "to jest" is not instrumental?
Yes, because Polish uses "to/to/tamto" while English uses "this/that/that".
But in this particular example it's actually a different thing. With such a construction (This is X) it will always be "To" no matter the gender or the number. So This is/That is (and also It is) will be translated as "To jest", while These are/Those are will be "To są".