I might have misunderstood the scope of the idempotency layer, I thought it was this thin layer close to the web server that just replays the last answer from the services that are lower down. So that means stripe actually goes down to some store or external system to check the transaction status for subsequent calls with the same key?

I know how complicated integrating with stuff like the bank, 3Dsecure, AML and fraud prevention is. One of the systems I integrated with had no refund functionality, they expected a manually initiated bank transfer from our customer bank to the end user! So I certainly understand some states are irrecoverable in a web request. It is important to do though, because it saves all clients work for each automated recovery that can be done.