Haven't read the full thread, so I may be repeating something already observed...
The poster didn't ask for Mumsnet to be compared to national averages, that's kind of the point.
Ok (haven't checked this, but saw on page 1) only 5% of the country have earnings exceeding 100k, but I think a reasonable percentage of those post on Mumsnet, which may give it a slightly more "middle class" tone.
5% of people in the UK may be in that top bracket. But for example, at a Polo club, 95% of the people are. I think Mumsnet, perhaps 30% of people are. And maybe another 20% purport to be, as it's the internet and you can "be who you like" in many respects.
I think it's that false 20% (again, just using figures for illustrative sense) that skew things. They make the genuine proportion look over exaggerated, so some posters disbelieve those who are genuinely in that situation, and those openly and honestly stating they are in lower brackets feel they are a minority.
I was a good earner. Many children later, I'm a housewife. I have a degree and another professional qualification. DH has 6 figure salary and had a reasonable commute, although this year changing so much for the workplace now means he does a lot based from home.
We are the Mumsnet stereotype. And I can't be alone. I see many posters describing similar set ups, and I personally find it frustrating that I am disbelieved when those are our genuine circumstances. "Suuuure you've got a degree" etc. Yes. I have. And I worked hard for it. "Suuuure, your DH earns all that." Yes. He does. It's not the be all and end all.
I like selling bits and pieces on eBay and find it quite lucrative. Something like that (for example) gets instantly questioned..."it doesn't add up that you sell things on eBay with your husband earning so much". Well, alas, that's how it is, it doesn't have to add up or be justified, it's the truth.
In the scheme of things it truly doesn't matter. I've lived a lot more lavishly than this, and in younger years been quite skint. I have posted on MN for so many years and always found posters to be helpful, constructively critical. You can have some ding dongs, but it's nearly always at an intelligent level, even if the opinions differ considerably. It's pretty straight talking, and not a babe/him culture. Long may it continue.