Restriction on working hours for full-time students
The extract you have linked is specifically for a program that exempts employers from paying the full minimum wage. There is, in principle, no law stating that students can only work 20 hours per week when school is in session. In fact, it's quite the opposite—students can work as many hours as they would like. 

On the other hand, it is possible for universities to place conditions on financial aid recipients that they not work more than some number of hours per week, and in general it's not really that smart of an idea to be studying full-time and working full-time on top of that. And the university may also have limits on what students can do with on-campus student aide positions, whether or not they are on financial aid.

But as far as the US government is concerned? I don't think they really care.