This is often how stuff starts. I don't have a lot of personal experience with social services but have plenty under the loving care of other services which have social workers and healthcare workers in them.
A worker (a GP, apparently, in the above example), who's overworked and stressed and probably not paying as much attention as they should, misinterprets the situation. In this case, they didn't understand that a family friend was the one actually accused by the daughter, but that social services decided that the 5 year old was more likely to be mistaken about which person was responsible than about the exact date it happened, and thought the blame must therefore lie with a family member.
In the quoted post, the worker decides that the mother disbelieves her child and defends her child's abuser.
(I accept you're not at work right now @Sarahcoggles; however, your post was illustrative.)
The way services seem to work, the rest follows from there:
Another worker, who trusts the first worker's judgement and professionalism, reads that the mother is trivialising the abuse and trying to protect the abuser, and takes the appropriate actions. The mother is confused, hasn't been told exactly why these things are happening, and is upset and doesn't want these "appropriate actions" to happen. So she's not engaging with the process, and this is written down, along with a copy of the accusations of abuse minimisation and abuser protection.
Someone eventually explains to her why these things are happening — she's minimised abuse and protected an abuser so she's a risk to her child. She argues back that she's done no such thing.
The previous report is copied over — mother disbelieves child, minimises abuse, continued to allow the family abuser to access her child for a while, didn't engage with the processes designed to protect her child, and now can be described as argumentative and combative.
Workers go into every interaction with her expecting a mother who's noncompliant, wilfully covers up abuse, and doesn't want to engage with services, and workers have the attitude you'd expect of them in that situation — and the mother, bewildered, reacts badly. By this time there's a mountain of evidence against her, any little thing she does wrong is catalogued by workers keen to ensure they have an the evidence they can about her failures.
And she still won't "admit" that her family member abused her child.
How does the mother get out of this nightmare? It's easy enough for it to happen.