I'm only going to uni in septemeber but I can already now say that in my case and in case of many of the people I know, friendships practically die during college. I my self had a lot of friends in school, a few of them I still had as friends in college since not all went to the same college as me. But most of these friends during my 3 college, slowly began becoming people I just know. At this point, college finished and all ready for uni, i don't have friends and I'm okay with that tbh. I do think it is due to the "only class mates" or "people to hang out at breaks" sort of friendship.
I think the reason some people may say that you start to make actual friends at uni is because after all, you go into such education at a much mature age (usually 18,19) and leave in your 20s. After so many years of education and meeting so many people, I think that at uni you pick your friends much more carefully than you did at school/college. You're then at an age where you spend more time with people outside of education so you're not seeing them as just "classmates" anymore but actual fiends you will see outside of uni, hang out in free time etc. You pick people who you know are going to be worth your time and not going to fade away really