Couple things you're not taking into account here.
A large amount of their money has always been collected in crypto. They likely had millions of $ of crytpo 5 years ago, which means they likely have billions worth now.
You are assuming that all their money is made through fee's, and although that is a very nice amount of money, I'm sure they were doing their own trading as well and probably making at least as much off of that as they were making in fees
I'd venture a guess as to say that they might have been earning a lot more via their own "trading"
If we could of course call that trading at all (since milking the clients can hardly count as trading). I traded at this exchange using their api, and you could query the server only once per two seconds (otherwise you would be receiving cashed data). This is in fact a tremendous amount of time within which they could be safely doing all sorts of illegal tricks (illegal for regulated exchanges, obviously) massively front-running the orders of their clients and squeezing profits from every other trade without ever revealing themselves