Gold is literally untraceable. Nobody can stop you from smelting and selling it as you like.
Gold is very traceable, because it leaves a trail of people behind. It's very heard to disconnect your identity from your gold sale, because you have to make that sale in person, so there are always witnesses, cameras, paper trails.
If you want to put in a middle man, you also need to know him to trust him not to run away with the gold and he becomes a liability, because he knows your identity.
That's why this is the maximum fungability you get, be it with bitcoin or gold, none of the two have the upper hand here.
And no, I don’t see how you can compare gold being “traceable” through detective work, to a ledger that literally stores every transaction in history, publicly.
Once a person trades coins at that exchange (perhaps even into a different cryptocurrency for additional obfuscation), then in the eyes of the other exchanges those coins will become disconnected from the activity they are trying to hide.
Once that happens, why would exchange X take your BTC, or any other people exchange other crypto for BTC there, if they can’t circulate it?
Gold has a real world trace that can be found out by simply asking intermediaries and numerous other means, Bitcoin has a digital trace, it's not that different.
It's very difficult to make sure that the history of where that gold you got came from and to whom you sold it to is 100% erased.