mdd, the Divrei Chaim didn’t rule on the Rebi specifically. He simply said that to deny that the Ohr HaChaim was written with Ruach Hakodesh, would be apikorsus. (Ruach HaKodesh does not by definition mean that everything someone says is right. It means that they get a certain level of siyata d’shmaya.)
kapusta, in your example, since the metzius is that he did not eat non-kosher, he simply avoided getting an aveira of eating treif. (Same with oomis’ example of the eiruv. He avoided getting the aveira of being mechalel Shabbos.) If he had gone to a treif restaurant, he would have then gotten an aveira of eating treif.