What he's saying is that everything should be idempotent, which may be possible for local only calls and filesystem snapshots, but anything doing a network call

What he's saying is that everything should be idempotent, which may be possible for local only calls and filesystem snapshots, but anything doing a network call is outside the realm of this possibility. Such a system would need to spin up a local, accurate backend for any network call, execute the call, verify the results are not catastrophic and retry with a real call, but then we also introduce time caused uncertainty as the real system may drift enough from the expected state during the local validation. A fun thought experiment but science fiction IMHO.