Being open source means absolutely nothing here because nobody has any way of knowing what the actual source code the centralized default coordinator is running.
Being open source means you can run your own coordinator.
It does as it successfully breaks the link between the bitcoins you convert to XMR and the bitcoins you get after selling XMR because of the way Monero is implemented.
I leave no trace other than the information that person with UTXO(s) x, y, z wants to trade them for either some cryptocurrency or fiat (specifically for what, is an information only known by the other person). Just as with coinjoin, I signal that I want UTXO(s) x, y, z to be mixed.
Monero is not the weak link in this equation, Bitcoin is. Step 1 of "Exchange your BTC for Monero" is where the privacy is lost since your Bitcoin transaction history is revealed to your exchange counterparty (unless you coinjoin to do this exchange, in which case you have no need for Monero in the first place since you already have privacy on Bitcoin).