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.
Sorry people were shitty about it. It's my favorite. Use it or don't, but please enjoy yourself no matter what you choose, and don't give That Site™ any traffic ever again. If you do that, everybody wins imo.