the fallacy just doesn't apply.
my definition is narrow, because unlike Scotsmen it is necessarily defined by a basic idea, (and why i included the Chambers and OED definitions), while Scotsmen are obvioulsy not wholly defined by their inclination to sweeten their porridge.
you're not even defining centrists. you're just characterising them, and in a way that contradicts 𝘢𝘯 accepted definition.