The quote comes from this Reuter's news article: http://www.reuters.com/article/2015/04/17/belgium-beer-bread-idUSL6N0WS2LW20150417. HotLicks is almost certainly correct when he/she suggests that 'thick' in this context is simply differentiating this bread from the very common flat breads known also from this time. It seems likely that the author of the article didn't quite understand the nuance, as he also refers to this bread being in 'loaves'. In some senses a thick flat bread is a loaf, and in other senses it is half-way to a loaf, but in both senses the sentence in the article is a little clumsy.
The point of the article is that the ancient Sumerians used this bread as a base for brewing beer, and the practice has been revived in several commercial breweries. The practice has, however, been commonplace in prisons for many years where inmates use crumbed bread to initiate illicit alcoholic brews.
As the OP points out, the bread is called 'bappir', and some detail is available here: https://en.wikipedia.org/wiki/Bappir, but a picture is worth a thousand words:
And here is a another image from a site discussing the link between the bread and brewing: