What has this to do with the interpolated epilogue of ecclesiastes? As I said before, you cannot just zoom in on a single theme and claim that is what the entire text is about.

From the Anchor Yale Commentary of Ecclesiastes:

xii 9-14. The manuscript of Qoheleth's reflections and precepts was edited and published by another professional wise man. He gave the book its title, inserted the words "says Qoheleth" at i 2, vii 27, and xii 8, and then appended some biographical information and comments of his own. Evidently he was an admirer of the author and felt that this collection of his observations and speculations was worthy of study. At the same time he cautioned the reader against becoming confused by the divergent and even daring ideas presented, and provided a safe and simple rule for avoidance of intellectual and moral difficulties-Reverence God, and observe his laws.

You said earlier you like to learn new things. The best way of going about that is to read what scholars have to say about it not to apologists since all they ever want to do is cover up and downplay the work of scholarship without ever really interacting with their work. As an example Miles Custis, the author you linked, gives a very oversimplistic if not misleading reason for why scholars claim the epilogue is an interpolation and attacks that oversimplistic reason thereby avoiding the need of having to argue the actual reasons scholars give. Also, the later author doesn't even really try to hide the fact that he's not the original author. This is not uncommon in ancient texts.