Looking at Swindon, the weekday timetable has four coaches a day between London, Swindon, Cheltenham and Gloucester, (three on Sundays), perhaps that's the new service that involves Swindon.

(Flixbus's web site's 'Network map' is actually a map of their destinations rather than their routes, so it takes a while to find that the route between say Cardiff and Swindon isn't quite sensible - it involves a change at Heathrow).

Mark