Even that is not proof. "I sold an old laptop to my friend. He wanted to pay me in bitcoin, so I gave him my deposit address for my Coinbase/Binance/whatever account. He paid the coins directly from the mixer to my account."

The bottom line is that as soon as a transaction has happened, you cannot say the bitcoin haven't changed hands. As soon as a transaction has more than 1 input and 1 output, you cannot say which bitcoin ended up where - indeed, "which" bitcoin is a concept which doesn't actually exist.

The very concept of taint is provable nonsense. It is a triumph of blockchain analysis companies that they have convinced governments to force centralized exchanges to pay them hefty fees to peddle this nonsense.