You've hit the nail on the head - note that Wollen bought his tickets on Sunday when the reservation system was down, temporarily removing advance purchase tickets from sale, which was clearly advertised on all the ticket sales websites I saw. Maybe the headling should be "Swindon Town fan didn't read notice talling him to wait until 1100 when advance tickets would be available again" but it doesn't have quite the same ring.
To be fair, the story makes no mention of Mr Wollen trying to buy the tickets online and it also says that the ^15.95 fare was a promotional
return including tube travel. And if Mr Harper was attempting to buy them by phone from
FGW▸ Telesales they may've been unaware of a local promotion. It is however difficult to work out from the story what the timeline is and where these promo tickets were available from. Can't see a combination of advance singles plus tube that would add up to ^15.95.