They mean its kept overly and unnecessarily complex, and difficult to understand on purpose. The reason being because the tax preperation industry lobbies to keep ot thst way because their entire industry is based on it being too complicated.

Byzantine became an adjective to describe a system that is overly complicated and difficult for a lay person to understand the inner workings of. The basis for the word was the bureaucracy of the Byzantine Empire - which was regarded as overly complex and impossible for people outside of it to actually understand. It was probably easier than the US tax code though