Is Pseudonymous Bosch really Jon Scieszka?

It is unknown who he is. Some names people believe he is:

Lemony Snicket (Daniel Handler)

Edie Bilmann

Jon Scieszka Raphael Simon

Morgan McDonald

Some believe he's actually a she.

Others hypothesize that a highly intelligent chimpanzee is responsible.

Others, noting that even a brilliant chimpanzee would be incapable of prose, have posited instead that the highly intelligent chimpanzee hired a roomful of chimpanzees with Word processing skills, and that those employees, in turn, alternate between producing the complete works of Shakespeare (completely useless to humanity at this point, since they've already been written) and the oeuvre of Pseudonymous Bosch.

We don't know.