For this set of much more relevant nonlinguistic systems, Vidale demonstrates that the average number of different signs is only about 44, a far cry from the 400 or so signs in the Indus script.
sproat, Liberman, and shalizi (in a blog by Liberman [2009]) and sproat at EMNLP'09 undertake the exercise of knocking down the straw man ("similarity in conditional entropy by itself implies language") and present artificial counterexamples (e.g., having Zipfian distribution) with conditional independence for bigrams (sproat 2010).