I don't see anything contradictory here. (I don't know what "call the operation queues" means, but maybe the word wasn't "call"? The video is behind a paywall, so I can't double-check.)
In the described scenario, none of the operations are running on the private serial dispatch queue. What (presumably) runs on the dispatch queue are one or more closures which invoke "OperationQueue.waitUntilAllOperationsAreFinished()".
In other words, the serial dispatch queue isn't for serializing the operations, it's for serializing the wait for operations to be done.
Ok. That makes sense. Thanks.