If you look at the map, it's really more of a case of one embankment and a couple of roads linking to the park being closed. Which is fine - just to say it's not the whole city.
I am not sure this was the best of ideas. Cars and bikes should be able to "co-exist" pretty well in cities...Shutting cars out just creates a nuisance for people who need cars for work or their disability. It doesn't prove anything. Its just rather token.
It's not really supposed to prove anything, it's a family fun day. Is the Notting Hill Carnival supposed to encourage people to dance to work?