Phead You're incorrectly implying that sandboxed Google Play has some special ability to communicate with other apps. They do not have any special privileges. That includes no special ability to communicate.
even though the Google apps are treated like any other app they can communicate through IPC
Apps within the same profile can communicate with mutual consent between the apps. For example, if you give the Contacts permission to one app, it could share access with another app within the same profile. It's supposed to enforce that the other app has the Contacts permission too but it may have a vulnerability or may ignore the permission model for nefarious reasons. This applies to any data you enter into apps and any form of access you grant including permissions. It is not something unique to Android. If you give data or access to software, it can do anything it's permitted to do with that data.
People often try to use the Network toggle as a way to toggle off data exfiltration but it's not the intended purpose. Apps could exfiltrate data by sharing it with other apps, either via intents (IPC) or access to shared resources. They could also do something like playing audio at a supersonic frequency since there's no permission for playing audio. Network toggle is a toggle for the app directly using the network itself or indirectly using it via APIs requiring Android's INTERNET permission such as DownloadManager or another service provided by a user installed app. As with any other permissions, an app could choose not to enforce the standard permission model, which is part of what you trust them to do when you trust them with access. In the case of the Network permission, it's not always clear when apps should require it such as browsers not requiring it for apps opening a link. This is already addressed by the option to use multiple profiles and GrapheneOS also intends to provide app communication controls within profiles eventually but it's quite difficult and has major caveats since users granting other forms of shared access would bypass it, such as granting 2 apps the Contacts permission or access to a shared file.