I am willing to accept if I'm wrong, that statement came from a friend who does work there, who told me all the other interns in London were Oxbridge. If I'm wrong, then please disregard the statement, but I'm yet to be convinced.
However to respond to a couple of points you made, firstly, I clearly mentioned the last few years, so not "the university origin of EVERY one of the dozens of people BCG has made offers to since, probably, before you knew of BCG's existence?". Secondly, that post was not intended to be "seen to be knoweldgeable, rather than to help people out", it was solely intended to help the OP in his decision of where to apply.
The general ethos of the post comes from before then though, from a discussion about grad recruitment possibilities with the head of HR from another MBB consultancy while doing national sponsorship for AIESEC, who mentioned that BCG particularly are Oxbridge centred and that it was quite normal for their entire graduate intake to be Oxbridge. Now, obviously being at a different MBB consultancy they may not have been as honest about it, but it seemed to fit with everything else I've been told.
I will point out this, like the OPs question, did solely relate to people joining their internship/graduate programme in London straight from uni, and not either experienced hires or MBAs joining as consultants. I am still willing to accept that I'm wrong, as I don't know the entire intake. However everything I've posted has been pieced together from talking with HR or friends who work at MBB consultancies.
Lastly, as a moderator I tend to post exactly as I did before - trying to help others to the best of my ability with what I know. Occasionally I am mistaken, and I will admit that a lot of what I say is taken from things people have said to me. Sometimes it may be wrong, but it's always designed to try and help answer the question at hand, and is not simply made-up speculation, but the product of things I have been told.