While the author makes a good point about how humans will never leave the solar system, there are a few possibilities to continue human legacy. Ethics aside, sending seeded capsules to a habitable exoplanet could one day (millions of years) evolve into an intelligent species. Although not human, a DNA signature or some other artifact could be engineered to validate it as a human colonization.

Additionally you could explore the possibilities of fleets of nano sized probes, which over the course of thousands of years, confirm the habitality of an exoplanet, and build a crude nursery for sperm (which could be sent at a later date). This method makes the energy/momentum problems slightly less impossible.