There is a phenomenon called 'blackfishing" that is fairly common on social media. A form of online catfishing, it is when people who are not black pretend to be black or black adjacent, to have black husbands and children etc in order to justify saying things that are essentially racist and anti-black.
It is common enough to make people wary.
So when someone sneers about "warriors against racism", and is on thread after thread minimising racism while claiming to be raising a black child, it naturally sets off alarms. Because the instinct if you have a black child is to fight racism, not sneer at those addressing it.
When that person acknowledges as their biggest supporter a poster who is known for troubling and untruthful posts about Meghan's mother, and who breezily says tone deaf things reducing race to just "skin colour", while claiming she is offended because Meghan called her racist, it gets murkier.
My experience with Mumsnet is that some posters generally get really agitated about being called out for the racist things they say. And they are very quick to see what they claim to be racism against them while minimising racism faced by black people.