I’m not sure if that is entirely true. If you use the Xano plugin which @jared.gibb could speak more informatively on, I believe it uses API calls in the client browser which Bubble I do think likely still charges for as an API call and data being returned from an API. However, I believe the major reason to use an external database like Xano, is not to bypass WUs for creating/modifying/deleting/fetching data, but more to bypass the WUs used in backend workflows that process the data as that can be handled by Xano.
You can schedule a paid session with me if you want to explore ways to optimize your Bubble application to reduce the WUs.
You could schedule via my training site or Bubble Coaching Marketplace