Transactions and positions are not all necessary super secret. People and other machines notice things. You need to take a lot of care to be unnoticed unless tiny and once you move to much it is basically impossible (might not know who you are, but might notice your actions and the impact).
Having info on someone’s trades doesn’t mean you know the basis for their trades. And by the time they have executed the trade any leaked info becomes less valuable.
If that were the case then adversarial HFT algorithms wouldn’t work.
