In 'Only a man is a donkey', the logical form of 'only' restricts the substitution class in ways that make the inference trivially valid due to the impossibility of the premise, not genuine formal validity.

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Substitution class(as used in formal logic): The set of things that can be swapped in for a variable in a logical statement without changing its basic structure or meaning.
formal validity(Medieval logic): A property of an inference whereby the conclusion follows from the premises in virtue of logical form alone, such that the inference holds under uniform substitution of its terms
inference(Nyāya epistemology): A component of epistemology in Nyāya philosophy; a veritable inference yields knowledge about the world and must have premises that are themselves known
logical form(Used to characterize logical consequence): The way that a sentence is built up from the logical particles.
trivially valid(The absurd consequence of applying standard validity conditions to arguments with truth-inapt premises.)

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