If you're using a catch-all route like that, the only way to make this work is to create routes for all other one-segment URI's.
You could hack your way around by using the 404 route. Don't define the route in the other post, so /admin maps to an existing controller and works, and /about doesn't. so it throws a 404.
Then, in your 404 action, do an HMVC call for your slug (i.e. do the lookup on 'about'), and if found show that page, and if not,set the 404 header and return an error.
The problem is that there is no automatic way do create a single route that maps /about to a page controller, and /admin to something else, without either one of the solutions I mentioned.
You should be able to access URI's like /admin/users/create, or doesn't that work either?