The idea that “shuttle” in this case means that at least some tickets for the flights would be flexible so as to be able to be changed or purchased until very close to departure — such as 10-15 minutes if presenting to the gate with no luggage to check — won’t fly.

The US Government/US DHS would be furious at DL/AF if they tried to make a habit of allowing that kind of close-in flexibility on departures to/from the US; the US DHS would very quickly put an end to it for the airlines.

Perhaps if AF and DL were to allow multiple flight confirmation bookings against a given ticket number, then perhaps they could engineer more of a close-in, walk-up type flexibility. But I doubt that US DHS would like that either.