Rational gave a good explanation. They were mistaken, since they believed something that happens to be heresy. The question you posed to him does not go against his point. It could be asked before he made that point. It is the next stage that you were waiting to put up. But the question of if those in earlier times are heretics was answered.
As to your next point, it is not as if we have two prevalent Shitos. There was an obscure view which was rebuffed all around by all the famous Rishonim. This applies to Reb Hillel as it applies to anonymous “Gedolim Mimenu’s.”
Besides, I take issue with the famous new idea that we can’t Pasken beliefs, which comes with many Yotzei Min Haklalim. The Maggid Shiur you mentioned who lambasts Daas Torah based on his Rebbi’s final decision (and who quotes his Rebbe reverently about all types of non-Torah topics), also holds that when it comes to the 13 principles there is a Psak, for some reason. Well, Moshicah is one of them.