The specific claim is that workflows are started exactly-once in response to an event. This is possible because starting a workflow is a database transaction, so we can guarantee that exactly one workflow is started per (for example) Kafka message.
For step processing, what you say is true--steps are restarted if they crash mid-execution, so they should be idempotent.
I think a clearer way to think about this is "at least once" message delivery plus idempotent workflow execution is effectively exactly-once event processing.
The DBOS workflow execution itself is idempotent (assume each step is idempotent). When DBOS starts a workflow, the "start" (workflow inputs) is durably logged first. If the app crashes, on restart, DBOS reloads from Postgres and resumes from the last completed step. Steps are checkpointed so they don't re-run once recorded.
"Exactly-Once Event Processing" is the headline claim - I actually missed the workflow starting bit. So what happens if the workflow fails? Does it get restarted (and so we have twice-started) or does the entire workflow just fail ? Which is probably better described as "at-most once event processing"