That sounds like bollocks.
More likely they had an expert psychiatrist compile a really in depth report on all his mental health issues over his lifetime leading up to the crisis that’s landed him in a mental hospital and someone has picked out the Oxford thing from that massive report and misrepresented it as Edwards trying to use that to get a lighter sentence.
My understanding of mental health being taken into sentencing is that the system is trying to give everyone the same sentence, and taking someone like Edwards out of a live in mental health hospital where he’s getting round the clock treatment and on suicide watch and chucking them in a prison is considered more impactful than doing it to a healthy person, just like being pregnant or having a serious physical health problem that can’t be treated properly in a prison would also be considered.
There also seems to be this false idea that the judge specifically identified his mental health as a reason he gave him that sentence, when in the sentencing remarks he just says he’s taken everything into consideration.
The bigger factor is probably that Edwards didn’t ask for or solicit the images Alex Williams sent him. The fact he wasn’t actively trying to procure this material obviously impacts how likely he is to reoffend and the danger he is to the public.