You highlight the results of intellectual growth, not the cause.
Lets take your example.
Two students are given an essay due a week from now. Student A completes the essay in a day. Student B completes it in a week. Both achieve a grade A.
According to you, Student A has greater intellectual capacity than Student B.
Would it be correct to assume that for Student A, it was the essay itself which gave rise to a greater intellectual capacity, causing Student A to complete it in a day?
Further, to you it, "would certainly count as intellectual development". It seems you are again confusing a result of intellectual development (i.e. completing an essay quickly) and the cause of intellectual development.
What caused the student to complete it in one day was their ability to rapidly consider multiple sides of an argument and come to a conclusion via sound points.
This process is fostered via 1on1 Socratic style learning at Oxbridge. If you cannot do this fundamental process, how would doing more essays (referring to more 'work') aid you? Yes, you
practise this process by doing essays, but you're not seeing the process and reasoning behind it. Thus, it's easy to say, more essays = greater intellectual growth.
In Math the concept is the same. You learn a set of rules from a given axiom, you then form an understanding of the rules by following their natural consequences.
This process is fostered via 1on1 Socratic style learning at Oxbridge. You then solidify this understanding by exploring specific instances of the rules (practise sheets/problems). Understandably, someone unaware of the process behind the learning will see: more practise sheets/problems = greater intellectual growth.
So, what gives Oxbidge the main factor in developing their students intellectually, than at any other university, is that they put a greater emphasis on 1on1
Socratic style teaching. As a result, students grasp concepts quicker, allowing them to cover more in the same 3-year span than students at other universities.
Put simply, when compared to other universities: It's the way you're educated that develops you intellectually; It's the way you're educated that allows you to process a higher workload. From my perspective, you're not seeing the greater process. I would google 'learning theory', 'socratic learning' and general psychology to fully understand what learning is and what it means to develop intellectually. It would be clearer then to see how Oxbridge fosters this.