I think that if it suited you (for some reason!) to think you had a specific illness, then you could infer that you had it from this letter, or use this letter as evidence you had it.
Or, if you were a catastrophiser or had chronic health anxiety, you could conclude, with horror, that you definitely had CBD and dash to google with rising panic, ignoring the letter’s caveats about how it’s not a firm diagnosis and that you clearly have a very mild and non-progressive version, and that most of the PSPA charity’s info won’t apply to you.
I suppose we can’t entirely rule out that the Walkers genuinely interpreted this consultant letter in the blackest possible light at the time.
But, given its temporal place in the series of RL events that were rearranged and massaged into the fictional timeline of TSP (ie. two years after the real house repossession, after two of the walking holidays the Walkers took during their 18 months at Anne’s, around the end of the time they spent living at SW’s mother’s house, just before TW embarked on a horticulture degree with the full blessing of his consultant, and also around the time they met the Australians at Fat Apples by which time they were claiming to be walking the entire SWCP, TW’s illness was in place in their self-presentation, but the homelessness angle wasn’t yet), it seems most likely to have been one of the things that struck either SW or both Walkers as a useful, heartstrings-plucking additional hook for her essentially fictional memoir.
ETA, I know someone has said this before on here, but it seems very likely to me that meeting the Parsons inspired the homelessness angle of TSP?
David Parsons had been working as a bricklayer when a back injury left him unable to work, they couldn’t keep up with their mortgage repayments, and they had been living in a caravan ever since, apart from stints of house or pet sitting, or cheap walking holidays like the one they were on. Not hard to imagine the Walkers thinking ‘Oh, that would be a good angle, if we shift things around a bit, and come up with a blameless reason for losing our house…’