Actually, he didn't send it. He has publicly announced that he created it to make a point but didn't actually submit it that way.
And none-the-less, he was totally off-base for blaming "common core". Common core is nothing more than guidelines for *what* should be taught not *how* it should be taught. People commonly (and incorrectly) blame the wrong thing. People have issue with how math is taught these days, but that isn't a direct cause of common core.