No, actually not. It is tied to your icloud or google account. When you buy a subscription in the app, you buy it from Apple and Google, and that particular icloud / google account has access to the premium subscription. This means that you can restore an in-app purchase on any device logged in with the same icloud/google account, no matter which Grosh account the app uses. Sounds a bit strange, but that is how in-app purchase technology works.