It’s a balance. On your output cap, typically a electrolytic with a parallel ceramic, a 20MHz signal at resonable amplitude would mean you were pushing a lot off current into the caps
So, 20MHz bandwidth since what you are measurering is not real, artifact from not 100% perfect probing.
Just be sure to not default to 20MHz, since you might be missing something in some cases. And, don’t take Davids words as the whole truth. He is not deep into power electronics, he’s more a generalist. He’s often winging it, but that can be ok if you know when he is