I taught quite a few sixth-formers who went on to either Oxford or Cambridge.
What distinguished them all was an ability to think logically and laterally right there and then. And they were capable of spontaneous flourishes of originality both in their essays and in classroom debate.
They tended to be open-minded and empathic too, intellectually curious about ideas and theories that they did not necessarily identify with in any way.
Truss, of course, exhibits none of the above attributes.
It is possible for someone to be bright but so slavishly in thrall to a particular belief system that they are incapable of seeing the wood for the trees. So maybe that has something to do with it in her case, an inability to be comfortable with ambiguity.
But I really am struggling to comprehend how such a talentless, morally repellent mediocrity has got as far as she has.