I think the key word there is "unlikely," referring to the position that the majority of Brits would find themselves in when taking the test (i.e. never held a full license before, anywhere).
Private insurance policies here usually allow one to drive a car belonging to somebody else, but almost always with the exception that this doesn't extend to a hired car. So even if you have insurance yourself, when you rent a car you're covered by the rental company's "any driver" insurance. As I don't know of any rental companies which allow learners to drive their vehicles, obviously their insurance doesn't cover anyone without a "full" license (and usually over 21, 25, etc. to boot).
The fact that somebody in your position already has a "full" license which is accepted by the company and thus covers you on their insurance changes things quite a bit, but as is often the case, I think the various people involved probably don't realize the implications because it's not something they've ever had to deal with.