An early copy of Ecclesiastes from about 175 BC is important for dating the book, not for proving faithful copying. There’s no earlier text to compare it to.

Both are true. The early date of 4QQoh-a refutes the older scholarly view that Ecclesiastes was written in the second century BC, which is a separate point worth having. For stability, the fragment is compared forward to the medieval text and agrees closely, and that thousand-year comparison is what this claim is about.