The Knowledge's purpose is not just to know the fastest travel between A and B in ideal conditions (although a 20% improvement in that situation would be nothing to sneeze at), it's to know it in any condition and to be able to find B however it's specified and to be able to explain landmarks to tourists. In fact the original article on the comparison notes at least part of that:

> The Uber leg does not start well. After the app is downloaded, it stubbornly refuses to acknowledge that the Guardian exists. Only with the help of an old-fashioned desktop computer is registration completed and a car booked for a four-mile journey from the Guardian's offices in King's Cross to Big Ben in Westminster.

and the difference in fare between Uber and the black cab was 8%.