I think he will have been diagnosed with CBD in good faith. It’s a bizarre thing to make up. Surely you’d pick a more common condition and scans and physical signs can’t be faked.
diagnosis is not exact in medicine and Drs can later turn out to be wrong and I expect that’s what happened here. I am sure they were told he had it and did expect him to die.
Just look at Stephen Hawking who survived an unfeasible amount of time for his diagnosis of MND (he was given 2 years and survived for 50!) but obviously was profoundly disabled and not putting it on
It may be that, as with Stephen Hawking, Moth was not re diagnosed because there was as no better fit