You can't trust the exchange to not inadvertently leak the information. A substantial leak could include personally identifying information along with account balances and or deposit and withdrawal totals.
Somewhere along to lines, if you want USD, you need to trust someone, it's inescapable. So once you know this, you can act accordingly.
But still, the original argument was that you have to worry about chain analysis because everything is public, but obviously that isn't true, no matter if an exchange may or maybe not inadvertently leak something.
