That's a serious oversimplification of the guard game, in my opinion, though it's a piece of advice I hear alot.
The best basic thing I always say to any one starting out is... you need to have 2 in or 2 out (arms), that is when your passing the guard, look at the first Nog vs Fedor fight and you can see a good example
If you're in the guard, dropping both arms out of the guard to scoop and pass is a good approach and, in that case, you definitely want to make sure you don't leave an arm in.
However, there's a huge gray area when it comes to leaving arms out in the guard.
Splitting the guard with a knee (either just popping it between the opponents hips and yours, or dropping it on the thigh to keep the guard open and make for an easy pass) allows the person in the guard to leave one hand on the opponents belt while using the other to pass over the leg, set up a navy ride or pass under the leg that's being underhooked.
I get frustrated by these little blunt pieces of advice that don't acknowledge the nuances of the ground game, especially when it comes to basics.