He is being a jerk. Also, not responding to the actual argument, as far as I can tell (which is technical, but quite clear). There are some other damning stat

He is being a jerk. Also, not responding to the actual argument, as far as I can tell (which is technical, but quite clear). There are some other damning statements in the rebuttal, too--for example, they say that all the new machinery developed for anabelian geometry lies unused in the case to which Corollary 3.12 applies (meaning most of the effort mathematicians have been spending in order to try to understand Mochizuki's new mathematics has been wasted). They also mention other incidental complexities that serve to obfuscate the argument but don't seem to buy anything (for example, not choosing a specific isomorphism at one point in the proof, but instead asking the reader to consider all isomorphisms with particular properties, for reasons he has been unable to justify). Someone in his position should be aiming to clean up his proofs to be as straightforward as possible, not making them deliberately harder to verify.