It is extrascriptural because the passage in Leviticus does not match the passage in Genesis 9 (uncovering vs. getting uncovered), and the passage in Genesis 9 contains enough references to physical actions (fatigue, drunkenness, sleep, covering while walking bakcwards, etc.) that militate against an euphemistically sexual reading and suggest literal slipping of a blanket or something like that.
Okay. If you want to believe Noah cursed a grandson, to be a servant of servants to his brethren because that boy's father saw him naked - okay. It isn't worth an argument.
....Ping