It’s well known and also wrong.
Delta’s airplanes also require a great deal of maintenance, and I’m sure they strive to have no more than are necessary for their objectives. But if you talk to one of Delta’s accountants, they will be happy to disabuse you of the notion that the planes are entered in the books as a liability.
It's possible for something to be both an asset and a potential liability, it isn't strictly one or the other.
Delta leases a big portion of its fleet, which makes your example pretty bad.
You're hinting at the underlying problem with the quote. "Asset" in the quote reads, at least to me, in the financial or accounting meaning of the term. "Liability" reads, again to me, in the sense of potential risk rather than the financial meaning. Its apples and oranges.