Based on the article and my background in dog training, I'd say Daisy qualifies as a service animal.
I also feel Delta needs to rethink it's policies. From the article:
If you see someone our shopping and have a dog wearing a Service Animal vest and you doubt the dog's ability, ask the the dog's owner to demonstrate some common, basic Service Animal commands like sit, lay down and stay.
Most people with an ESA dog, sometimes called a Companion Animal, will be able to have their dog perform these commands. Oh, and be sure to remind the person that a real Service Animal animal can do these commands with the command spoken just once and without any help from the owner.
You can also visit this link to learn more about the training standards for service animals
“If you see someone our shopping and have a dog wearing a Service Animal vest and you doubt the dog’s ability, ask the the dog’s owner to demonstrate some common, basic Service Animal commands like sit, lay down and stay.”
And under ADA they can tell you to GFY. Especially if you are just some interested bystander.