Americans can't buy the ticket. EU citizens only.

Source? I seriosly doubt that. Yes, it's difficult to find a suitable seller, but it's neither forbidden nor impossible.

I only have anecdata and I don't feel like googling until I find something that confirms the real life experience.

It's probably not impossible, but it's also not as simple a transaction as it could be. It is a difficult affair, at least bordering on impossible unless you're a EU citizen with a SEPA bank account.

Citizenship is definitely not relevant. Owning a SEPA account is a precondition at most sellers, even a German SEPA account at some, although the latter has been decided unlawful by courts before.

However, another comment somewhere mentions a seller that accepts Paypal. I'm sure I have heard about others accepting credit card. They are rare, but they exist. It doesn't matter where in Germany you buy the ticket, it's always valid everywhere. Buying the smart card version at some weird place might be practically infeasible and sometimes they are not issued on the spot but after a delivery time. But for buying the app version you "just" have to do your homework. Quick, easy, and convenient, no. Possible, yes.