Oh I see the point o your question now. He was identified as the prophet Isaih in the book of Isaih. So again yes, in writing anyway he was being called a prophet.
(didnt read the whole thing, yeah, I know) Its just seemed out of place to find a seal from a letter with the inscription “Isaiah the prophet” and attribute the seal to Isaiah’s time unless he was commonly known as that at that time.
Its still very early, here in my head.