The article implies that there are more transactions going on than would be expected for an exchange with lots of user accounts. The first diagram suggests that there is a rotation of transactions, i.e. coins are churned through multiple addresses many times. That kind of thing is not needed to operate an exchange; you just need to gather money from deposits, send money to withdrawals (many exchanges fail at this point :) and shuffle the rest between a hot wallet and an offline store from time to time. In other words, the ethereum transactions are far in excess of what a typical exchange would need. A bitcoin exchange doing so many transactions would probably go bust from the fees alone!

(However, the story isn't crystal clear about what they mean by the 'rotation' of transactions. It says 'rotated 80 times' but I'm not sure if that means each input address goes through 80 transactions before going to an output.)