Doxxic change can always be coinjoined later, and before that it is marked as unspendable, meaning you cannot harm your privacy accidentally by consolidating it with mixed funds.
You're right that tx0 is not a coinjoin and therefore *every* transaction cannot be a coinjoin. What is meant by that mantra is you should coinjoin every utxo you have, and use the post-mix tools offered by Samourai and Sparrow wallets. I don't trust wasabi because I have seen graphs of mixes where each utxo is not mixed with a unique partner, making it much easier to trace mixed funds back to the original owner.