I can well believe that FS, with her low IQ and high compliance, would have stood by and let SB harm her little girl. The Milgram experiment showed how often people are willing to comply with perceived authority figures, even if that results in harm to someone else, and, to my knowledge, the people involved in that experiment were neurotypical.
It's so sad to think of how different Star's short life could have been if SB had been a kindly person who wanted to nurture instead of to control and destroy - I get the impression that FS would follow someone because they move, frankly, and wouldn't need to believe that they're kind as long as she thought they were right and it is tragic that the person she met and attached herself to was such a destructive, malevolent influence.
My DD was born two months before Star was and I've followed this case closely because there are photos where they look so similar, and it broke my heart to think of that little girl, who should be doing the same things as my baby, suffering so much instead of being loved and doted on. I wondered during the trial if the FS's defence strategy was to make her appear as stupid as possible (like when she asked what 'contrast' meant, and when SB's barrister asked her if she knew what a pronoun was and she said no), so I found her IQ test result very believable. I think the verdict is right; I think she did allow Star to be killed through her inaction and willingness to ignore or cover for SB's behaviour, but I do think that her low IQ and behavioural traits mitigate when I bear in mind how many NT people go along with bullying or causing harm because they believe that the person doing it or causing them to do it has authority, somehow.