He said life, not humans. Taking a life-cycle analysis, biological entities are dramatically more efficient than existing robots. Designed a nanobot that gathers energy and resources from the natural environment to reproduce itself? Congratulations, you just invented the cell.

It's also worth noting that our robots are far from perfectly adapted to space. Usually, they get to the destination by jettisoning all manner of fuel tanks, landing gear, etc, and then they die in a few years. They're adapted to being put together in factories on Earth. Much like we don't have isolated livable environments on Earth, we don't have self-maintaining robot factories, either, and both problems seem pretty hard.

I think a compromise solution might be developed over the next fifty million years or so. No idea what it's going to look like.