Considering the distances involved across the milky way Galaxy I question that estimate of a time to colonize the entire Galaxy.
What? What? Are you saying 'warp drive' might only exist in science fiction novels?
“Considering the distances involved across the milky way Galaxy I question that estimate of a time to colonize the entire Galaxy.”
... in addition the study presupposes aliens WANT to colonize an entire galaxy. An advanced alien society might find all the resources it needs within its star system and see no reason to travel further. Given the enormous distances it might involve less resources to terraform and adapt bodies within a star system as their civilization grows than for a handful of their people to travel to another star system where conditions may not be any better than their own star system.
There’s a good chance that there are many planets with all kinds of life, but interstellar transit is a practical impossibility. If you believe in a creator you have to wonder why he would make such a huge universe and only fill one tiny part of it with life.