While I've not walked any of it, and so cannot scrutinise aspects of the route, I am very familiar with long distance walking and camping generally.
I posted a while back an inconsistency with the book chronology and real chronology, particularly around the fact that it can be deduced in the book that they stop the first leg in early October, meanwhile we know they were driven to Bristol on the 17th Sept. This creates a two week inconsistency, suggesting they either started earlier than August as they claim, more likely end of July close to when TW was surfing in Newquay, or they walked far quicker than they claimed.
We also have the inconsistency of the play they claimed to have watched weeks earlier on the path, yet in reality the play opened on the 16th Sept.
The Simon Armitage thing bothers me too. SA started his walk at the end of August according to publicity. Meanwhile Raymoth claim their walk started on a Thurday afternoon in August. If we are still believe they started in August, the aforementioned 2 week discrepancy means we must assume it to be the first Thursday which was the 6th Aug (other wise the time discrepancy would be 3 or more weeks).
Getting back to SA - while his plan to walk the SWCP was publicised in national media that year, I find it a stretch to believe people began to mistake Moth for him (irrelevant of the dubious likeness) so soon near the start of the Raymoth journey, at least two weeks before SA began. Would SA really have been so prominent in local people's minds as to even consider they'd seen him? And if so, they'd have known he was starting later due to whatever media publicity they'd seen.
LDW also requires a level of experience and despite their claims of having done lots of camping in their 20s, they come across as completely clueless in TSP, though perhaps that was for dramatic effect. At one point SW writes of guilt of getting a bus as if it's cheating. But cheating what? The feat of walking the entirety of the path or cheating Moth for denying him some extreme physio?
My best guess is they likely did the journey, but quicker than they claimed, probably by a combination of walking the easier bits quicker and getting buses/taxis to avoid some of the harder bits. Could they have even used a bag transfer company?...not sure such exists for SWCP but they do on other national trails. And I don't think anyone ever mistook TW for SA other than something they joked about between themselves.