From an accounting standpoint, SkyMiles are definitely a liability on their Balance Sheet. When award tickets are booked (or maybe when the award flights are actually flown), then DL recognizes revenue (moving the liability to the Income Statement).
You're absolutely correct - SkyPesos are represented as deferred revenue on the balance sheet, which is a liability, but I also see his point. It's not like you can just one day decide to make your bank loan go *poof*, whereas DL could theoretically do this to the liability represented by SkyPesos (though it certainly wouldn't be that easy or clean).