I don't know where you have read that, but it is incorrect. This is taken from the list of Healthy Extras on the Slimming World website and counts as a B choice:
Probably because it is an 800g loaf. I think the other HExB wholemeal breads that are two slices are two slices from a 400g loaf, which is equal to one slice from an 800g loaf
I've just checked and one slice is a hexb but to syn it depends upon the type. They are anywhere between 5 and 6.5 as there are different types, original, wholemeal, soft and so on.