Not quite.  It doesn't cover the case where a route is a string prefix of the base path, but not actually under the base path.

eg with a base_path of `/vat`, it should not be possible to add a route for `/vat-rates` (`/vat/rates` and `/vat.json` should be allowed though).  See 2nd paragraph here: https://github.com/alphagov/content-store/blob/master/doc/input_examples/route_registration.md