Are you stupid? The backend is completely different and there are many many changes required since the fundamental concept of federation-based services is different and requires different user decisions, which need a UI. No instances on Reddit, are there? So there’s not an infinite number of log in urls, there is no need to gracefully handle defederation, instance-level blocking, etc.
It’s not a copy paste job.
Also, he offers a “restore purchases” button. It’s in settings.