Extending the quote somewhat, it reads
Defy Danger is for those times when it seems like you clearly should be rolling, but no other move applies. Defy Danger also applies when you make another move despite danger not covered by that move.
I've been reading this as "when the inherent danger in what you're trying to accomplish isn't built into a more obvious move, you're going to need to DD first to get a chance to accomplish that move". As in:
GM: "Hey Nekhbet, the scorpion-men are stabbing you with their vicious hooked swords and poison stingers. Their leader is asking you questions about where you hid the treasure."
Nekhbet: "Okay I'm gonna Defy Danger and cast Jigsaw Organ Condition on the leader so his lungs fall out and he dies of suffocation."
GM: "Okay, first you have to Defy Danger, the danger being their stingers. Roll it."
Nekhbet: [rolls, gets a 7-9] "Shit, that complicates things, no?"
GM: "Right, so, you ramble off the first couple of syllables, but then spot a stinger racing towards your face. You crouch down, it misses you, you quickly scurry out of the way of the next one, but your concentration is lost. Their leader starts waving this wand made out of a human skull-and-spine towards your general direction and mumbling in some guttural language you don't recognize. What do you do now?"
In other words, the GM recognizes that Nekhbet shouldn't be hurt or receive any other imminent major consequences of failure, because he did succeed at defying the immediate danger. However, the GM is now putting Nekhbet in a spot - he is once again under threat by the stingers, and now additionally threatened by this unknown curse their leader is concocting. If he proceeds with casting his spell now, he will get pierced by those stingers; if he evades the danger of the stingers, he will get afflicted by the curse. Hopefully he has a friend nearby who can help with at least one of those (such as Defending him from the stingers), else he's pretty much fucked.
On a 10+, Nekhbet would've ducked around the stingers, completing the verbal and somatic components of the spell and getting a chance to roll to properly Cast it (with yet more risk of failure or half-hits, as Ludanto points out).
On a 6-, perhaps Nekhbet would've gotten hurt by the stingers (the obvious choice), or become overrun and pinned, or the spell backfired due to the distraction.
That's how I'm reading it, at least. I'm not sure how else you would run this situation - just tell Nekhbet that if he attempts anything but Defy Danger, Hack & Slash, or Defend, he's just going to fail in the end due to the imminent melee danger he's in?