Well, that’s at least the premise. Some manage not to. 2500 hours per year times 4 years is 10,000 hours. The opportunity is there; whether many squeak thru without taking proper advantage of it is a different issue.
That may work for a science major who has to do problem sets each week. Humanities students who only have to write a paper and take two exams per course per semester may learn a subject (more or less), but don’t get much experience writing, revising, and seeing comments on their work. As writing is one marketable skill they could have acquired, college may not have been worthwhile for them.