In your Google scope this all works out however if you're a mature but not yet prevalent, neobank whose reputation rides almost solely on availability, I'd still say theirs is a great approach. Even in the unlikely event that AWS throws in the towel and has a hard failure, users can still enjoy a subset of functions without elevated latency. On top of it, it might well be that user data can never leave UK, so geographical redundancy might not even be a thing here.

And yes you are completely right a completely distinct implementation is probably a no-go, but I'd think you could deploy a similar codebase with feature flags, with some frontend differences. I'm speculating but pretty sure they didn't reimplement backend functionality for this.