Yeah, Android's sandbox deals with low-level file access and socket APIs and stuff. But Android also, intentionally, allows apps to expose their data and functionality to other apps via the higher-level "intent" system.

Apps get to choose what permissions are needed to access their intents, so under Android's security model, really it's Chrome's fault (or whatever browser the user has installed) for exposing an intent that allows apps that don't have the INTERNET permission to exfoltrate data.

Similarly, apps are also allowed to collude to share data with each other if they want; that's how stuff like Google Play Services works.

As I understand it, this "Application Sandbox" is to prevent app A seeing what app B is doing.

The problem here is a different one: The user granting an app access to their files and then having no way to prevent that app from sending that data to the developer of the app.