BUT that argument either begs the question of what is objective, or is based on metaphysical notions of objectivity.
Introspection is to be rejected because it is especially prone to error.
BUT introspection, when done properly, can be as objective as any other kind of observation. Introspection is to be rejected because it is unreliable. Therefore, the link between introspective verbal reports and internal stimuli is unreliable. There is a public-private distinction. Introspection is to be rejected because introspective evidence and observations are private, i.e., available only to one observer, and are thus not objective.