rl - acknowledgement of error where its due: you're right about the context of the Josephus remark - sorry 'bout that; I lost track of which discussion was at discusion.
My comment that Josephus does not provide provenance for Isaiah stands (as does my observation that you misrepresent to suit your agenda - but thats another matter); the Isaiah tradition would have been part of Josephus' religiocultural heritage, something he'd have neither reason nor basis to question, any more than you question the scripture you hold sacred. Anyhow, at particular reference were the purported prophecies of Isaiah, which led to the issue of that work deriving from multiple authors over a span of centuries. In that discussion, it was developed that that the majority consensus of legitimate biblical and historical scholars - presented through compelling, objective analysis - is that the 66 chapters of Isaiah comprise the work of multiple authors and redactors, its writing spanning a period from the 8th Century BCE into the 5th Century BCE, if not later.
You are welcome, of course, to dispute that, endorsing the minority position held all but exclusively by biblical literalists. A number of mutually corroborative mainstream academic and theologic references were provided, clearly establishing the majority consensus; you choose to not maccept them - fine, your choice. For a thoroughly Christian perspective on the matter, I refer you to
The Unity and Authorship of Isaiah: A Needless Battle, by Dennis Brachter, Ordained Minister, PhD (Biblical Studies), Director of The Christian Resource Institute, and respected, prolific, widely cited, author of many books and articles of Biblical Studies and general Christian Apologetics.