Very interesting. Similar to thoughts I've had on the topic. Complex life, especially motile animal life (well beyond the simpler eukaryote development) may be very rare. As the authors say, main sequence lifetime is much longer than stable habitable zone ecosystem lifetime. Not enough time to jump all the hurdles, on the average. The latest rage postulating habitability of M star zones needs to be calibrated by the extreme NARROWNESS of those HZs. Forgeting the 'minor' problem of radiation flares, any HZ (liquid water under a few tens degrees C) planet has to be exactly in the one narrow right spot.

The overly simplistic view popularized by Sagan et. al. that just because there are billions and billions of stars in the Milky Way means that statistically there are millions of technical civilizations is becomming as outmoded as the late 19th century view of our galactic environs.