There is definitely something in this. I have no skin in the game as my DC1 is not applying for Oxbridge, but I know friends' kids who are and when looking at our selective, high performing independent school, there are every year a dozen or so applicants with straight 9s and 4 x A*s who don't get an offer or often not even an interview, despite competing in national maths/computing etc exams and scoring very highly.
I just had a look and the ones from more recent memory who did make it (on competitive areas e.g. computing, engineering) all had the following in common:
4-5 x A stars (for single science e.g. Chemistry I think some had Astar Astar AA - contextual/ethnicity)
11-12 x 9s
Top 15-20 nationally in certain competitions (e.g. maths Olympiads etc), taking part and reaching final stages of global engineering competitions.
Most would have probably been top scoring (top 180 or similar) in regional/country grammar exams.
Many were 'Head Boy/Girl' and academic + something else (e.g. Music, Sports, Drama) scholar
All I'd say would be extremely hard workers with not much of a social or extracurricular (i.e. not referring to supra curricular) life.
Saying that, I do think if you're from a comprehensive, the grades required would (understandably) not necessarily be as above but lower / contextual.
I think the competition is fierce out there currently and I do feel that there must be some real brilliant minds that just won't get selected as the bar is so high and the work you need to put in is relentless. I do hope that admissions professors somehow look at 'potential' too as if you work to a very high level academically, how much room is there to grow - some of these kids working all hours must absolutely have hit their ceiling of potential? However, this must be very difficult to assess even with interviews now - entrance tests help - as there is so much information and courses you can attend to get interview help.
I guess many unis, and Oxford more than Cambridge, place emphasis on 'achieved' grades (which for most are GCSEs) as predictions are generally so inaccurate. I think around 80% (if not higher) of UCAS predictions nationally are inaccurate, and usually upwards!
Oxbridge won't be right for everyone. There is something to be said about enjoying your university life, growing as a person socially without that intense pressure which would allow this to happen and for the student to easily fit in extracurricular stuff, work and socialising. Of course, there are some students out there (I guess the top 1%) with exceptional minds and memories who can combine both but, for most, I think Oxbridge is a place where you just have to work extremely intensely and putting in long, long hours (short terms, mind!).