Muslims believe and at the same time, reject the idea that their quran was written by humans. They want to accord their quran the highest ultra divine status, so the idea that the quran was written by humans is blasphemous. Yet, the hard truth is that the quran was never compiled in the lifetime of their founder muhammad. It was compiled decades after his death, from a scattering of his speeches written down by various people at that time. You can treat it like a canonization of his speeches, just like how the various books of the Bible were assembled by the leading priests of the day to form today's Bible.