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 provably 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.
Of course you can't and that is a perfectly logical explanation. Generally, the clause of the exchange's TOS forbids the association of these tainted coins. Even if you were to argue from this narrative, they have the rights to not serve and allow you to trade on their platform.
Bitcoin is not fungible to them but what can you do? You have two options, build your case by intentionally running your coins through multiple hops and come up with an easier and more believable story or use another exchange if you'd like.
Bitcoin is supposed to be fungible but the key issue is that they could care less about closing one account so long as they conform to the regulations.