What I do doubt is that all these schools are in areas where people routinely have money for tutoring and private school fees (which is the classic mumsnet take).
But it isn't necessary for all these schools to be in areas where people routinely have money, in order for a handful of people out of the thousands who use Mumsnet, and the hundreds (probably thousands) who read thread titles in AIBU to have experienced them. It's only necessary for some of them to be. And the 22% figure I quoted indicates that there is a reasonable likelihood that there will be some, which is why I quoted it.
My area is a market town which is is pretty socially mixed, which draws pupils from the surrounding farms and villages as well. There are both people who can afford tutoring, and people who can't. There are some who can afford private schools, and many who cannot. This situation is not that unusual in places other than the Southeast and large cities.
I really, really resent people implying that I am lying when I say that in my opinion the school is not good, (and I will be very surprised if this is not reflected in the GCSE results next year once controlled assessments are got rid of).
Also I disagree that it's the 'classic' mumsnet take. People who clearly live in areas with sink comprehensives, who could never afford private schools, also post about their poor schools, and also get their experiences dismissed on these threads. (Not surprisingly, they tend to stop arguing and go away.) And a fair number of people don't say whether they could afford private schools or not.
I admire your passion to support comprehensives, but implying people are lying about their local situation doesn't do the cause any favours, in my opinion. If people feel their experiences are being dismissed as lies, they are less likely to believe it is possible that problems with comrprehensives will be addressed, IMO, and more likely to turn to alternative ideas like grammars.