It's not tautological because it's objectively wrong.

There is no biological purpose. We are just a multitude of chemical reactions falling down the entropy slope in a slightly odd way.

"Purpose" is a concept entirely made up out of whole cloth for hairless apes who accidentally invented language and that allowed them to do a lot of thinking outside the confines of their biological processes.

We are perfectly free to CHOOSE reproduction as a "purpose", but nothing about biology actually cares. If you don't reproduce, oh well, something else will take that spot in the ecosystem.

That's the most plausible gloss I've heard. The new generation must be capable of framing a question at least as sophisticated as its parents can if there's to be hope of progress in the world. Otherwise we're on the way to "Idiocracy".