I'm pretty sure there no direct flights from Iran (for instance) to the US, so this doesn't would seem in theory (and practice) to be applying to anybody from the 7 countries. Iranian PhD student stopped in Frankfurt - for example.
it will be to cover directly booked through flights where the individuals don't clear security at the transit airport.
Existing airline systems don't send API/PNR messages to the transit airport for transit passengers , so their security services don't get informed. Instead the information is sent to the final destination airport, in this case the DHS et al.
Where the passengers disembark and clear security and check-in again they will be relying on the transit nation security services to take action.