sorry, but I think this is a bit misleading.
It's absolutely true to say that you shouldn't worry too much about college choice. In the end, they are all part of the same University, they are more similar than they are different and the vast majority of students will end up perfectly happy with and loyal to their choice.
However, it would give the wrong impression to say that the reason it makes little difference is because students can access each other's bars and (some) events, as if this is all a college offers. A college provides many things that are only available to students at its college.
A college provides tutorial teaching. The tutors are different across colleges, there will be different styles and (I'm afraid) different amounts of teaching, and different shares of teaching done by graduate students and other temporary staff. The balance between university and college teaching (and exchanges between colleges) varies from subject to subject, but the college always matters.
A college provides accommodation and meals, of different qualities and prices (and a few don't provide it for the whole degree).
A college provides all sorts of welfare support, financial support, sports facilities, music facilities, common room membership, other college clubs and societies, college only events.
Most students do most of their socialising within their college.