So.. what they're pointing out is that you could BCC and then quarantine the BCC into a "test" policy quarantine, so you can see how quickly you'd fill up space based on your real mail flow, but without actually disturbing your real mail flow.
A common question new ESA users ask "well how many mail can my quarantines store?" "how much space is enough" etc... which all depends on what your actual mail is. And this is a way to get an idea of what you need.