What surprises me about the whole Oxford fiasco is I've always been under the impression that private schools, particularly the really reputable ones, basically groom students to ace the Oxbridge interview/application process in a way that state schools never do. I'd be really surprised if Ruby didn't have dedicated lessons on how to get into those unis, either within school or with private tutors. So the fact she messed up the interview so badly despite her massively privileged education really just shows how much she DOESN'T belong at a university like that.
[tl;dr of this is needlessly personal information about my experience of an Oxford application, feel free to skip haha]
I don't know anything about the private school system, but I was lucky enough to get support from my school when I applied to Oxford (the same year as Ruby, coincidentally). A lot of it was encouragement to think out loud if I didn't know the answer to a question and articulate each step in my thought process (instead of just saying 'um, uuuh, hmmmm' before I arrived at a full and complete answer, which I would have done otherwise!). I think it's likely that Ruby never practised this skill, particularly with respect to literature.
I also watched a huge number of example interviews online - not just for my subject - which hammered it into my head that I wasn't going to ace every single answer for every single question, so when I did trip up in my interview, it didn't completely throw me off. I imagine that once she got caught up in her own head about it, she couldn't quite get back on track. I don't think her school prepared her for it to not go smoothly, even though few people have a perfect interview.
tl;dr - I think there's only so much that schools can
teach you, in the way that Ruby absorbed knowledge and then repeated it back. The preperation takes practise, making mistakes, doing badly, trying again, getting better - a process I'm sure she wasn't familiar with.
I'm sure the system is different in private schools, but as has already been stated, Oxbridge look for people who are passionate and thoughtful beyond a syllabus. Ruby was probably praised all her life for being book-smart, and I don't think she ever would have reflected on how different an Oxford interview would be.