They both screwed up, we know tadashi only gets turned on by Tsubame which, if we go by the codes of the romance genre, means that he has some feelings for her. He most likely said that he hated her on the spur of the moment because he panicked when he heard that she was willing to be his gf (if he truly hated her, he wouldn't have sewn back her button in the first episode). So we will probably learn that he regret what he did in the next chapter and the whole drama will be resolved and buried.