He might have attended the school of General Studies at Columbia, and not Columbia College. GS was frequented mainly by part-time students taking night classes, students who were able to maintain a certain amount of anonymity, as they didn’t live on campus and most of them commuted. To put it as delicately as I can, the admission requirements for GC were much more relaxed than for Columbia College.
I was at GS at the time and in poly sci. I took all graduate level courses as an undergraduate. GS students were totally anonymous and generally 2nd class citizens at Columbia. And yes the requirements to get into GS were pretty easy.
I believe too that Obama went to Columbia; that likely he was there as a GS student and that he got in as as foreign exchange student. That makes best sense of the info given.
He has mentioned that he was still paying off college loans in his 40’s as was I.