Exactly. An easy way to think about solving the exactly-once problem, on top of an at-least-once message layer, is to use the at-least-once layer to write idem

Exactly. An easy way to think about solving the exactly-once problem, on top of an at-least-once message layer, is to use the at-least-once layer to write idempotently (eg, HTTP PUT) into a sequence of numbered messages. There is no possibility that the receiver will execute message 18 before message 17. Each PUT retries until an acknowledgment is received, and the sender maintains a sliding window of sequence numbers it's trying to write. All the Two Generals problem tells you, in this situation, is that the sender does not know whether the receiver got the last message. It's an important issue and I agree with the general sentiment of the OP, but perhaps it can be overstated.