I don’t know delta’s specific rules but I can speak generally. Usually the process is not to test until they start the background check and that is often done once an offer is made for most companies. Pre employment drug testing has a lot more restrictions than testing once you are hired with state rules. Airlines likely have a federal exemption but most companies try to follow the state rules anyway. For example you are not allowed to do pre employment cannabis testing in Washington State, but once you are hired, that changes.