Don't believe him. Read it for yourself.
There is a fragment in it written by Noah. It is not complete, but is included within that book because it came down through the flood, through Noah, and Shem, to Abraham -from Noah and Shem, and to Moses, eventually.
Noah had visions like Enoch and had revelations like Enoch did.
Interesting that before the flood, the gate to Eden -the Mount of God that Adam got cast down and out of- was accessible to those on earth, which is also testified to in the Chinese Oracle Bone writing, in that the Border Sacrfice offered once, yearly, to the Creator by the Emperor for thousands of years, was at first depicted offered at the "gate to the Garden, a Mount"....and the prayer is almost identical to that offered once, yearly, in the Atonement sacrifice of ancient ISrael.
So when Noah was born, he was so shining and white and spoke in the arms of the midwife, praising God, that his father thought his wife may have messed with the Watcher angels who fell and had supernatural offspring.
He ran to his father, Methusaleh, who ran to the Gate of Eden, where Enoch was dwelling with the angels for those many years, and cried out for Enoch to hear him.
Enoch did hear, and Enoch came to him and told him the truth about Noah and his future.
Methusaleh went to Lamech and told him what Enoch said, and Noah grew up knowing the story.
In the Dead Sea Scrolls in the that which is called the Genesis Apocrypha and which includes fragments of many of the patriarchs own stories, there is a section written by Abram when he told Sarah to say she was his sister when they went into Egypt, for fear he would be killed because she was so beautiful. She did, but God warned the Pharaoah and after some time, Sarah was returned to him by the wise men of Pharaoh who asked him to teach them "wisdom, values, and truth", and Abram read to them from the Book of Enoch.
That happened within less than 35 years after the flood, and Abram had lived 39 years with Noah and Shem learning the ways and law of God while hiding from Nimrod, who wanted him dead. Abram came out from Noah and Shem and back to his father's house immediately after the tower of Babel fell and the tongues were divided. Abram knew who YHWH was, and he taught the Truth about God to all and was a teacher of righteousness all his life.
The information I just related is in the true history of the patriarchs called "the Upright Record/Book of Jasher", and the DSS manuscript scrolls have proof fragments in them that indeed show the patriarchs all wrote their own stories, which were used in redacted form for Jasher.