It may be the longest distance recorded for a continual flight by any land bird, but it's not an extraordinary performance.

For instance, common swifts can flight continuously for months. They obviously accumulate a huge distance during this time, even when they're not migrating. They live in Eurasia, from Lisbon to Vladivostok. And they winter in Africa, south of Congo. I think Vladivostok-Harare is a longer trip than Alaska-Australia, but it's probably hard to put sensors on small birds during their migration.