We are talking about generation ships so one lifetime isn’t a limit. Second, they said no unmodified life would make it. Third, I wouldn’t be so quick to rule out the Copernican principal when it comes to rocky planets in the Goldilocks zone, you’re just making the same mistake that those who said any planets were a rare event and they were very wrong. Small planets are hard to spot and most of the ones we’ve found have short orbital periods around small stars. Using transits for detection is also very limiting because the orbital plane has to align perfectly with ours. Hell, there was just a story about the JWT finding a bunch of Jupiter sized planets in interstellar space that nobody predicted so it’s looking like planets are everywhere more abundant than we thought.