What you just explained is correct, if someone did exactly what you said there is no problem.

I suppose that @Kruw said if you merge some mix or coin-join transaction with another transaction that hasn't been mixed before.

Let's say that you don't have enough experience with all this topic, but you want to enter in this (i am talking of a newbie user).

Example: you have some mix of sources in your UTXOs, some of them from mixing/coin-join and some other from a KYC source like some CEX.
In that case if the user mixes some of those KYC UTXOs with some coin-joined UTXO then all the previous work becomes useless.

I remember reading somewhere before that all the "chain analysis" that some companies do is not based on any reliable document or research. It is basically something like "I believe that those Utxos belong to the same person/entity"