This is pretty much what the article tells us, in more general terms: "a sixth appears to have been used to receive Bitcoin from another cryptocurrency exchang

This is pretty much what the article tells us, in more general terms: "a sixth appears to have been used to receive Bitcoin from another cryptocurrency exchange" Customers doesn't directly transfer funds to cold wallets (if they were they would be much easier to identify) and there's very little other reason to intermingle customer funds with proceeds from trading on other exchanges.