I had to check my Strava, and it's been a long time since I was near 20mph average on my "commute segment", over a year, so I have to say it's very unusual for me, but I know that a strong rider could surpass that easily. So I disagree with the thought that people don't really do it, or are exaggerating.

What makes it almost meaningless for "average commute speed" is that my segment chops off the parking lots and driveway at both ends, which can be 3-6 minutes on any given commute, which makes more difference in the door-to-door speed than how hard I try to hammer it. Plus, whether I catch the left turn signal green and how many times I slow to pass by people, but more than anything just the ends can knock the "average" down 2 or 3 mph.