The issue is precision in terminology and resulting understanding. At-least-once delivery with idempotency imposes certain rules on the processing of messages. As you say, every distributed systems engineer knows this.
When you call something exactly-once, people who are perhaps not distributed systems engineers make the reasonable assumption that this means exactly what it says. They will engineer around this reasonable assumption based on a clear technical description and get something hilariously broken in non-obvious ways. This will have happened because jargon ("exactly-once delivery") has been confused for a technical description of a delivery system's properties.
Not everyone in this series of comments is a distributed systems engineer. Never mind everyone using a messaging system.
When you call something exactly-once, people who are perhaps not distributed systems engineers make the reasonable assumption that this means exactly what it says. They will engineer around this reasonable assumption based on a clear technical description and get something hilariously broken in non-obvious ways. This will have happened because jargon ("exactly-once delivery") has been confused for a technical description of a delivery system's properties.
Not everyone in this series of comments is a distributed systems engineer. Never mind everyone using a messaging system.