According to the docs, the transaction release timeout is specified by the requester. The requester also specifies the duration to wait between retries and the

According to the docs, the transaction release timeout is specified by the requester. The requester also specifies the duration to wait between retries and the maximum number of retries. So this does indeed appear to be exactly-once as long as the sender ensures the transaction release timeout is greater than the total time (retry timeout * max retries) plus any network delay.