I didn't think they could, here's a couple of photos of the packaging which isn't very clear, it states 100% cocoa but cocoa powder contains carbs.
Actually, the latest NICE guidelines [2015] also carry this same advice.It was added as section 1.3.8 in [2009].The only piece of good and accurate advice I got from my diabetes nurse when I was first diagnosed was to avoid foods labelled in any way as diabetic. So sometimes they can get things right!
Robbity