The specific app that this thread was originally about, appears to contact the Play Store to simply download an addon.
Apps do this for the banal reason of reducing the initial download size of the app from the Play Store. This specific app contains several add-ons, which would make the initial download for the user a few hundred megabytes larger if they were initially bundled together, while the user may not want to use all the add-ons anyway.
Users are more likely to download an app that is smaller in size. Think an app that is 500 MB with all add-ons included versus an app that is 200 MB without addons. Not all users have beefy connections, or unlimited data usage.
It's normal that apps do this through Play Feature Delivery, or Play Asset Delivery for games. These are documented developer resources. I doubt that this case is an example of a sneaky collusion between the app and the Play Store. Although it is always technically possible, as several users have pointed out.
In all honesty, and in general terms, it doesn't make sense to me to give an app sensitive data, and at the same time clearly not trusting the app to abuse that data (such as selling it off to data brokers). You might use the network toggle with the intention to block the app from sending away your sensitive data. But even if the app doesn't use IPC to communicate with other apps, what's to say that you don't accidentally, while intending to grant the network permission to a different app, accidentally grant network permission for the app you don't trust just because you had an inattentive moment? The app could've just collected the data locally and then send it off to the cloud when it finally got a network connection. The app doesn't need to know about the existence of a user-facing network toggle in GrapheneOS, and try to overcome it specifically, because turning off the network toggle simply makes the app believe that the network is actually down.
I've observed in the past that this is a controversial statement. Which might be due to how I've been communicating. It's my own reflections. The point about being inattentive, for all I know might only apply to me, as I'm often inattentive as heck.