Exactly once delivery is not the same as exactly once processing.

Exactly once delivery is an impossibility; see the two generals problem. Exactly once processing is usually at-least-once delivery coupled with deduplication or an otherwise idempotent action

You CAN have "at most once" processing, since there is always the option of processing zero messages due to systems being down, but it's a joint effort, nobody is able to offer it "for free" (as in, no effort to implement it)