I kinda wondered that one too. It seems, from reading the article, like it is just a piece of carving with a small cross on it. A chess piece is just one possibility.
Another supposition, headlined as fact, in order to attract.
Hey, they got me to read it!
The Byzantine Empire was quite civilized and by the 10th century...90% of the population in Constantinople could read and write...unlike Western Europe which was in the throes of the dark ages.
I am not surprised that they would be playing chess in the Byzantine controlled areas.