Defining “myth” is essential, as is “real history”. In a technical sense, “myth” is used for a story with cultural significance beyond just being a story. The Bible certainly includes elements of that. However, in popular usage, “myth” is used to mean “made up”.
The Bible is not “history” in the sense of a history textbook about Europe in the 1800’s. Much that would interest a modern historian is omitted. As the postscript to John highlights, the historical incidents recorded in the Bible are not intended to provide an account of the history of ancient Israel, but rather are selected to illustrate theological points. Details that make an interesting story or provide some historical context but do not have specific theological intent may be included, but the purpose is conveying the theological message. This is why, for example, Kings and Chronicles often have a different-sounding picture of the same ruler. Kings generally gives an overall assessment; Chronicles is more likely to pick out individual incidents where a particular king might sound better or worse than typical for the reign overall. The Bible is an authentic ancient Near Eastern document. It talks about real people in real places. But it summarizes and interprets selected events from that history, and it uses literary devices in conveying its message. It uses the writing conventions of the times and places where it was written. For example, many conversations in the Bible were not written down while the conversation happened as a stenographer would, and may be summaries, paraphrases, or inferred guesses as to the gist of what must have been said. Important information (such as prophecies) were typically written down in the ancient Near East. Given that people then were no better than us at heeding instruction, messages were likely given multiple times, not necessarily in exactly the same words. What we have preserved conveys the message rather than necessarily a verbatim transcript of a single occasion.
Making the entire Old Testament exilic to post-exilic, or insisting that a particular theological concept must be “late”, does not seem much different from making the entire geological record fit a young earth model; the data are forced into an assumed time frame rather than taken seriously. The exile was certainly quite influential theologically, but only because the Jews already had an established and written (though often not heeded) theological tradition that provided a center of continuity. Other deported nations disappeared, merging into the cultures where they found themselves. But exile was a familiar hazard of living in the ancient Near East, and various individuals experienced it at many points prior to the fall of Jerusalem to the Babylonians. Mention of exile is no proof of being written after 600 BC. Theology certainly developed over time. But people have all sorts of ideas all the time. There is no fixed trajectory of thought.