Rather, it's a compiled lament by Solomon (Jewish king and son of the Jewish king David) about the futility/emptiness of a life lived apart from devout devotion to his god, and the stuff in the book is described later as "many proverbs" that were "arranged with great care" (Ecclesiastes 12:9), a description that comes after a long paragraph by that supposed-atheist that starts
>Remember also your Creator in the days of your youth, before the evil days come and the years draw near of which you will say, “I have no pleasure in them”;
and just before a paragraph ending in
>The end of the matter; all has been heard. Fear God and keep his commandments, for this is the whole duty of man. For God will bring every deed into judgment, with every secret thing, whether good or evil.
Not exactly "a rant by an...atheist".