I'm guessing it's being called extraordinary, because the godwits are flapping continually rather than gliding to take a break, from article:

Unlike albatross or other long-flying seabirds, godwits are active flyers, not gliders—their wings are moving the whole time. “It just beggars belief, really,” Riegen says. “I mean, though I've known that for decades now, I still find it hard to imagine how anything can keep up that sort of effort 24-hours a day, without taking a break.”