I haven't seen it directly mentioned here, but the attack behind this lawsuit is one of the major reasons Delta last year overhauled their ESA policy. Carriers didn't want to deal with threats of lawsuits from people who were loudly trumpeting their "rights" with their ESAs, so they initially had fairly loose restrictions concerning the rules for what constitutes an ESA. The combination of people masquerading their pets as ESAs so they wouldn't have to pay the pet fee combined with a number of other "incidents" (many of which later required deep cabin cleaning - ew) got the airlines rethinking this approach, but this attack was the straw that broke the camel's back. When Delta rolled out the new, more restrictive regulations last year, this attack was specifically cited.
I don't know to what degree Delta will ultimately be held responsible. The airlines aren't fans of ESAs to begin with, but they're required by law to accommodate (even beyond what the ADA requires), and I imagine Delta's response will be that they were complying with federal law as broadly understood by the airline industry. They may also add that they were leaders in getting the legal interpretation changed as a direct result of these attacks.
Going from memory, here, I seem to recall there was ample evidence at the time (or, rather, in immediate retrospect) that the large dog was not, in fact, an ESA, but the owner played the ESA card so they wouldn't have to pay the pet fee or abide by pet restrictions.