Given that there are objective changes, it is not unreasonable to believe his claim that he is satisfied or has benefitted from them.
The objective measurements are about his enhanced abilities. He can do things he couldn't before.
But, the GP comment referred to "quality of life" which is innately difficult to measure objectively. It's possible that he was able to do those things but it caused him enough irritation to do them that he avoided using it (like CPAP often is for example), or that the things it enabled him to do weren't sufficient to warrant feeling improved. My father has limited mobility, but no interest in playing mario kart or adjusting an air filter, and there's very little in his home that he has or would want to be automated. Anything that could be my mom or another family member usually takes care of anyway, even if it's still something he could do himself as he's rather tech illiterate.
So, in this scenario, given my father's age, the risks involved in such a major surgery for his age, and his personal inclinations, the very same additional capabilities likely wouldn't be worthwhile in his opinion. Hence, the subjective experience of the objective changes are how you measure quality of life for this kind of operation.
In any case, just like the stock market, the fact he responded well does not guarantee someone else will.
What we need is more data, not a higher degree of confidence in this one point. An independent review would be nice to satisfy our curiosity, but it wouldn't add much to our understanding anyway.