I think the bus maybe a misplaced asset from the end of the game. I have found some vehicles used in completed past missions elsewhere on the map. One I found was Judy’s van at the end of a dead end street and another was V’s fixed nomad car sitting in the petrochem parking lot. If you find one save the game then reload the game it removes it most of the time. Sometimes assets need to be preexisting on the map somewhere before they can be used in scenes.
You can leave Night city on a bus in Johnny's ending.
So the asset exists.