Then you might want to update the Wikipedia article which claims that it's a fallacy "in which one modifies a prior claim in response to a counterexample by asserting the counterexample is excluded by definition", citing three links to support that: https://iep.utm.edu/fallacy/, http://www.fallacyfiles.org/scotsman.html, and https://archive.org/details/godphilosophy0000flew/page/104/m....

To say "No true Scotsman would dislike haggis" is to assert "If A, then B": If you are a true Scotsman, then you will like haggis. The response "Angus doesn't like haggis" is asserting "not B". To which the response "therefore he's not a true Scotsman" is asserting "not A". But "if A, then B" logically implies "if not B, then not A". Therefore when the person's definitions don't change, *it is not a fallacy*. It might be wrong — his definition of a "true" Scotsman might be a false premise — but the conclusion logically follows from the premise, so it is not a fallacy.