The grades were just so that you could ‘matriculate’ ie be admitted into the university - you just had to have at least 2 A level passes.
I heard that several times, in different contexts, but never saw it written down, so it may be a common myth. However if you did well enough in the entrance exam and interview to be offered a place it’s very likely you’d get reasonable A level grades.
DH took the Cambridge entrance exam in the early 70s. At that time students either took it in their fourth or seventh term of sixth form. Students who were accepted in their seventh term and who were successful then had a ‘gap’ two terms working, travelling the world or whatever.
It was reasonably common, at least it happened in both boys’ grammar schools I knew about, to have an ‘express’ stream. So, pupils did their O levels a year early, after four years at secondary school not five. Applying for university in the year after they’d taken their A levels meant they were 18 when they started at university, despite having had time out between school and college.
Digression:
Don’t forget, the Cambridge colleges were single-sex for undergraduates until the first men’s colleges started taking women in 1972. Until then there were only three colleges which took women and about 26 which took men. It might be justifiable to think it was a lot harder for women to get into Cambridge than it was for men.
In my seven years at the girls’ grammar one girl got into Oxford and nobody into Cambridge.