He says "by" twice. You can also check the subtitles/captions track.
If "past couple of centuries" is 2-4 hundred years then that would suggest interstellar travel as early as 1996 if Tarbolde was from Earth. The old DY-100's were supposedly interplanetary only. Perhaps there were other ship types that were sent to Canopus/Canopius and by 2018 faster impulse engines came into being?
MITCHELL: My love has wings. Slender, feathered things with grace in upswept curve and tapered tip. The Nightingale Woman, written by... by Tarbolde on the Canopius planet back in 1996. It's funny you picked that one, Doctor.
MITCHELL: That's one of the most passionate love sonnets of the past couple of centuries.