I think it's more that they're the kind of people for whom there's only ever one side of any story, theirs -- and that other people's clashing or competing narratives must be aggressively fended off and exploded, because The Truth Is Theirs. Meaning other people are, always, just plain wrong, whether they're Chloe Hajimatheou or the Treen campsite manager or the person who wouldn't refill their waterbottles.
I think we've all met these people in the flesh. They also show up a lot on Mn on AIBU, looking for validation, often in high dudgeon about some perceived slight or another person who has made it clear that they regarded the OP as being in the wrong over some issue of parking, or hedge height, or managing toddler behaviour on a slide. It's why AIBU is so perversely fascinating, because these people genuinely believe themselves wronged, and are incredulous there might be some other position.
The Walkers probably genuinely think the uncle who lent SW the money to repay the Hemmingses 'betrayed' them by securing the debt against their house, or selling on the debt, or even by expecting them to repay it in the first place, because, hey, he had way more money, and it wasn't fair, anyway. Mistakes Were Made.
I think the outrage in the representation of the 'Cooper' courtcase is probably entirely authentic, even if the circumstances are fictional.
SW goes to a lot of trouble to position Raynor and Tim as the authentic, brave 'little people', free spirits sticking it to the man, whether that's by sneaking into campsites, or bravely defending themselves in court, or being snide about people who get to eat a plate of food each in a cafe, sleep in a B and B in bad weather, and get highlights.
In the most neutral situation, including meeting some friendly fellow backpackers in a nice cafe, she will find a way to create a power imbalance, positioning the Walkers as the lovable underdogs.
At Fat Apples (probably because, if we believe the Parsons' timeline, it's 2015, the Walkers are no longer even notionally homeless and are in much the same position as the Australians, walking for leisure) they pay to camp in the FA wood campsite and buy themselves 'a vast plate of vegetarian joy' in the cafe. SW then needs to find a way of not letting the reader think 'How luxurious', so she makes the Australians into figures of luxury, with two breakfasts, a budget for hotels in bad weather, and highlights that need maintaining. Compared to this, the Walkers' fiver for a campsite and a shared plate of food is small beer, and she draws attention to their continual hunger and her crappy, thin sleeping bag.
And this bit struck me:
It was hard to leave the sanctuary of the Fat Apples; I could have spent the winter in their wood, using the cold tap in their outside toilet, but we probably wouldn’t have been good for business.
She implies that she knows, magically, if she'd asked for them to stay on, the FA people would have refused because they don't want a scruffy, homeless couple camping in their wood and using their outside tap. So although no one at FA has ever suggested for a moment they were unwelcome, or refused them water, cold or hot, or charged more than a very reasonable £5 for a night's camping, SW manages to turn this into a (fictional) rejection on financial grounds. A nicer campsite, she seems to imply, would have begged them to stay all winter, regardless of 'business'.
I have no problem believing that she now thinks they've been unfairly smeared by Big Journalism.