I'm going to have to disappoint you, yellow, in that while the circumstances i describe are different from yours, that does not make them myths. Any more than I would stoop to call yours myths, since what you describe is not merely the admissions requirements. The entry requirements are clearly set out, I think yellow's DCs have demonstrated they have gone beyond a mere 3 A2s.
Yes often five ASs include CT/GS, but not for, say, medics who are very talented elsewhere. [For avoidance of doubt, I'm not saying spotting a good medic has anything to do with other talents, just that so many seem to.] Five A2s are not rare outside the UK, even in the 1970s. Oxbridge colleges aim for the top few centiles in ability worldwide and it is rare that those students would be satisfied by only the bare academic requirements while at sixth form.
Yellow your DCs know that they have very short terms and the ability to produce quality and quantity, and to play hard, too, is essential. This is not about A2 count, but about the type of student that will fit and thrive.
Students chase whatever they are interested in for themselves. Trying to second guess admissions tutors is not going to get you up in the morning to follow that 4th A2 or 'conquer' that piece of work, but your interest will. So 4 non-CT/GS A2s or their equivalent may not be commonplace, but we are not talking commonplace.
I've also said there may be some who find themselves ready only after year 13. The common thread is sound self awareness, helped along by good school support and discussing any questions with current Oxbridge undergraduates, especially ones from your school, and ever helpful admissions tutors.
Incidentally it is naive or disingenuous to say double maths is less than two A2s when Cambridge would be and has been happy to take on 3 A2s including double maths if you get through their interviews and you've also managed a few BMOs respectably and do spectacularly at STEP - which is massively more difficult that Oxford's MAT. It will have been said many times that a talented mathematician can get through double maths in no time, but the fact is most people struggle with Maths let alone double maths. Mathematicians mature earlier, generally, so that ability is more obvious than in the humanities.
I also find it surprising that prospective medics may not be aware of the early clinical exposure London medical schools offer, which some prefer over Oxford's course. I don't think the students we know ruled it out since they also received offers from Oxford. That of all options is one where frankly whether you go to Oxbridge or any other on the Avicenna list, you'll still be called 'doctor' in the UK, How good a 'doctor' you become has little to do with whether you went to Oxbridge. So it boils down to which course you wish to buy, assuming you can be bothered to distinguish between what's on offer.
At the risk of boring myself, I will repeat that admissions tutors are generally very helpful. They also tend not to be held back by crude extensions of a few candidates' profiles, since they've got more experience of who will fit. In the final analysis, they just want the best who will be successful after their 3-6 years, not who got in at the start. A short phone call may be all OP needs.