Passengers generally have the option to alight/re-join at Bournemouth when using RTC's Dorset Coast Express (et al) or Swanage Belle tours. On the former, it's not unknown for part fares between Bournemouth and Weymouth but there is the (generally) diesel hauled return which puts off some people. Just because you quote "hauled between Weymouth and Southampton" doesn't preclude a stop at Bournemouth. Nowhere does it state it is non-stop between those stations. RTC have used the reversal at Southampton for many years and, as you presume, they travel via Romsey and Laverstock before regaining the mainline at Worting Junction and then follow the outward route. Most passengers, I suspect, just want their train to drop them off at the same station they boarded and wouldn't be unduly concerned which route was used to accomplish this nor necessarily the semantics involved in the route description. As we know - some of RTC's tour descriptions have, in the past, been the subject of much forum chatter!