By definition, then, no true Scotsman (Christian) could possibly disagree with him on core beliefs, because anyone who disagrees with him is by definition not a true Christian.
I might mention here that there are two reasons why the divisions among Christians are inconsistent with the Gospel being true. In the first place, it shows that the Holy Spirit is failing in His alleged mission to maintain unity and doctrinal purity in the Body of Christ, the Church. Jesus’s analogy was that no one can rob a strong man’s house if the homeowner is alert and watching for burglars; you have to subdue the homeowner before you can rob his house. For Satan to be able to get in and sow dissention and succeed, he would have to first somehow subdue and bind the Holy Spirit, which he ought not to be able to do.
But the second reason, and the more important reason, why divisions are so devastating to the Gospel is that God does not show up in real life to tell us who the “true Scotsmen (Christians)” really are. It’s all well and good to argue that men are to blame for the divisions in the Church, and that your own group has somehow been miraculously blessed with the privilege and responsibility of being the only ones to hold on to the True Gospel when everyone else fell away. But every group claims to be the group that held on to the One True faith, and that the other groups are the ones that have fallen away. In God’s continuous and universal absence, men have no choice but to pick whichever group follows the doctrines (or interpretations of Scripture) that seem right in their own eyes—a practice that even the Bible condemns as unwise. Because God does not show up in real life, the believer cannot know that his group is the right group. He can only know that his group is the one he agrees with the most.