United is effectively saying that they will not provide refunds for cancelled flights, only for significant delays, because cancelled flights do not exist. They are treating DOT rules – which specifically provide for two separate reasons for a refund – as though they only provide for one.
If an airline used to fly 8 times a day between two cities, and now flies just 3 times a day, that airline has cancelled 5 flights even though the airline can still accommodate the customer. United disagrees, but that is absurd on its face.