I thought convention in SI units was to drop down a level when at less than 1 of a unit..?

Um, 1 billionth of a meter is 1 nm. Saying it's 200 nm (like the paper says) is very different from saying it's 1 billionth of a meter (like the linked article incorrectly says).

Though it's almost unheard of now, a british billion = 1000 * a british million. In that context it does hold, being 2 british billionths of a metre thick.

That said, here we all bascially use the American 'billion' because it makes a more logical approach and allows for dealing with larger numbers more easily.

Decent point, but most people familiar with metric would work with smaller units than metres and for those unfamiliar with metric, describing it as a billionth of a yard would still sound like a pretty peculiar.

I think describing it as a minute fraction of an inch or centimetre would still be pretty accessible without having to get people to visualize a nanometre.

Right, I thought the usual layperson speak was to compare it to the thickness of human hair or a spider web.