Then the flight captain will demand that they disembark (literally: "put off the plane").

Yes, thank you!

Yes, disembarked like from a ship. A caiptain gives an order, he does not demand, because his word is law onboard. A more correct Danish sentence could be: "Så beordrer flyets kaptajn dem (sat) af flyet", could be what confused you in the first place :)

Set down as disembark?

The answer could also be: In that case, the airline captain requires them set down from the plane.