General Studies students can take courses that are offered in Columbia College with permission and if that course is necessary for their course of study but not offered in GS.
So it is possible that a GS student could have taken a College course and be remembered by that professor—but not be in the regular student body of Columbia College.
Note that the Columbia College article refers to Obama as an alumnus. It never states that Obama is a graduate of Columbia College. Your comments about taking a College course or two while a General Studies student fit that to a tee.
The implication is that the writer(s) knew full well that Obama did not graduate from Columbia College, and chose their words precisely to avoid saying it.