First, in my post I was saying that an article published by an tutor can't be taken as evidence of the process, because no one would ever be as stupid to say publicly that they treated people unfairly.
Second, there is a whole weight of evidence that discrimination happens in all selection processes. A good proportion of this is when people do not think they are discriminating. A certain amount of this can be trained out of people, but if it was possible to eliminate it all, we would not have the inequality on the basis of gender, ethnicity and class in organizations that actually try to operate equal opportunities policies.
Whilst I am sure that many of the Oxbridge tutors try to be fair, that does not mean that discrimination doesn't happen. I am also equally sure that it would be possible for prejudiced tutors to disguise their discrimination in academic reasons.
In other words, Oxbridge interviews will be subject to exactly the same issues as any other form of selection process (eg employment).
As an admissions tutor, we work solely on AS grades and predictions. We largely ignore personal statements and references which are much more likely to be related to the social background of the applicant rather than their aptitude. I always try to be fair, but I also fully accept that our selection process is not perfect, and some applicants will not be offered a place through indirect discriminatory practices.
I am very suspicious of any admissions tutor that claims that this would never happen, as this suggest to me that they actually have no idea how indirect discrimination operates.