Fundamentally Oxbridge entry has never been based on academic results directly as you're implying. You needed near-perfect grades to be considered, but the vast majority of people applying with those grades fail the interviews regardless.

The interview is absolutely the primary test here, with the grades just acting as a filter to provide a manageable number of applicants. Widening that filter to allow more disadvantaged students the chance to interview seems perfectly reasonable - given that the interview itself remains equally demanding (and I've seen no suggestion or evidence against this).