Portmore does have a doctorate. Therefore, Portmore is licensed to practice medicine. Validity is a hypothetical notion. To say that an argument is valid is only to say that if the premises are all true, then conclusion must also be true. Validity is an all or nothing sort of thing.
Arguments are either valid or invalid.
In determining whether or not an argument is valid you don't have to know anything about the actual world. All epistemologists have read Descartes' Meditations on First Philosophy. Anthony Brueckner is an epistemologist. Anthony Brueckner has read Descartes' Meditations on First Philosophy. Not all sound arguments are good arguments.