From what I understand, Duffel are connecting to NDC APIs exposed by the airlines (or rather, by the Airlines' IT provider, i.e. Sabre and Amadeus... but that's another story). And on top of that, they expose a nice REST API (that is very similar in philosophy and structure to the NDC XML - Offer, Order). This is a win for the airlines, because every booking through Duffel is a booking without paying the GDS tax