It's likely that a vast majority of their volume is fake. Why would you fake it to add 10%? In the absence of real numbers, treating it as 0 is likely more accurate than taking it at face value.
It would certainly be interesting to see an analysis of BitMEX.
Unregulated exchanges fake volume to stay competitive with the other unregulated exchanges. Treating those as 0 means you may as well say "only 5% of exchange volume is traded on regulated exchanges, and we don't have the slightest clue about the rest."
It would certainly be interesting to see an analysis of BitMEX.