Trades don't take place on the blockchain, instead majority of bitcoin trading takes place on centralized exchanges.
Exactly, whales trade on the OTC (Over The Counter) desks, seldom associated with an exchange.
The whale deposits his bitcoin on an exchange, the OTC desks trades with an opposite interest outside of the books visible by anyone, then the whale withdraw the stablecoin resulting from the sell (of course the opposite side of the trade does the opposite transaction).
So, while the magic happens outside of the blockchain, we still see the deposit/withdraw at the exchange.
I am extremely skeptical of any statements by anyone saying that "whales did this" or "whales did that". It is impossible to accurately associate addresses with people or companies just by analyzing the blockchain. Even with heuristics and advanced surveillance techniques, I don't believe anyone can be as accurate as they want you to believe.
Of course you are right: every blockchain analysis does an heuristic-based deduction on what is visible on the public ledger.
This is also the very reason why every blockchain analysis based is deeply flawed (Taproot is going to potentially fix that).