The "sugar bible" does not say that fruit juice is out because of a lack of fibre. The reason that fruit juice is so bad is that it concentrates the fructose from 6-8 pieces of fruit into one serve, as well as removing the fibre. So you get a huge hit of fructose in one go.
The point being that you are better off eating one piece of whole fruit, and drinking water.
So adding fibre to fruit juice makes about the same sense as adding it to soft drink. It sounds like the sort of thing manufacturers come up with to convince us to buy an unhealthy product.