Where by "de-anonymize" they actually mean "detect what region the user's Apple ID is set to". The fingerprinting value of this is pretty weak, especially since, in most cases, it'll be the same as the country inferred from the user's IP.
It's also trivial for Apple to mitigate if they consider it to be an issue -- they can make the banner appear regardless of whether the app is available to the user.
Your Apple ID country has a lot of implications if changed (certain app access will be available/restricted, payment methods, etc). You aren't going to be able to identify a user specifically, but you could detect if a user is currently traveling internationally or may be a frequent VPN user.
The problem is that combining the two would actually give you significantly more information than either individual datapoint. A mismatch between Apple ID region and IP region will very reliably identify a subset of users (either expats or travellers).
No, the banner shows up either way and the size of the banner doesn't change based on whether the app is installed or not. So there's no way to determine whether or not the app is installed using this method.
Where by "de-anonymize" they actually mean "detect what region the user's Apple ID is set to". The fingerprinting value of this is pretty weak, especially since, in most cases, it'll be the same as the country inferred from the user's IP.
It's also trivial for Apple to mitigate if they consider it to be an issue -- they can make the banner appear regardless of whether the app is available to the user.