The headline numbers were mixed, but management's commentary on industry dynamics was bullish.The devil is in the details. Despite some mixed-headline numbers from Delta Air Lines' (DAL 0.31%) latest investor update, one aspect of management's commentary is bullish for the company and much of the industry. Here's an unpacking of what management says and why you can feel more comfortable holding the stock afterward.Mixed-headline numbersFor those investors unaware of summer events, airlines suffered flight d ...