I had a poke around on a hiking Reddit, which was being quite technical about whether people thought the Winns had genuinely walked the SWCP or not.
It featured a lot of unexpectedly fun insights into fake hikers, including a link to Youtube videos by someone who claimed to be the first teenager to solo hike Land's End to John O'Groats (which among those in the know is known by the acronym LEJOG, which tickles me). The Reddit thread is full of hilarious nuggets about how this kid's hair remains freshly cut throughout the 39 days he supposedly hiked, his blinding white runners and socks remain blinding white (yes, he's not even wearing hiking boots), he never mentions route, food or equipment, his rucksack moves like it's stuffed with newspaper when he lifts an arm, and he is apparently is walking in the wrong direction (away from John O'Groats) in several videos. 
(The hikers of Reddit conclude that he walked and camped for a night or two, then gave up, drove the route and periodically got out of the car to take photos and film whenever there was an appropriately lonely and scenic-looking background, generally near a main road or stopping point. Apparently there's one video where he talks about not having seen anyone for the whole day's hike, it's so remote, but there's actually a busy car park just off the A82 just out of shot. 😀)
Back to the Winns and the SWCP, while I'd certainly always assumed they had walked most of the path, several commenters on that Reddit, several of whom have walked the SWCP, find hardest to swallow that two people, even fit and well, could walk the whole thing on the inadequate diet RW says they ate, or to credit so many people being terribly surprised that 'old' people are long-distance hiking, as so many ramblers/LD walkers are middle-aged that it wouldn't strike anyone as all that unusual.