A 20 minute mile doesn't include waiting for walk signals at pretty much every intersection on the route, and the Manhattan distance between the Caltrain station and his office might be larger.
Yeah, I'm just going on his statement that it was "one mile", which I take to be the actual Manhattan distance, not the crow's distance. You have a good point with the walk signals, but as someone who's spent a fair amount of time walking many miles around Manhattan (but not SF, so I don't know about that), I never spent much time waiting for walk signals. With the grid layout, it's pretty easy to take a route that has you catching the walk signal almost all the time in one direction, and then opportunistically cross the street or change direction when you happen to catch a red light in that direction.