And that's true. It is arrogant. These two properties are two important to overlook. However, if we do overlook them, coins sent to ChipMixer were not connected via blockchain transactions, and that greatly increases the anonymity set, because the analyzer has to take into account all UTXO in the blockchain with amounts 1 mBTC, 2 mBTC, 4 mBTC etc. This is clearly bigger set than all the outputs of a coinjoin.
There's nothing particularly wrong that ChipMixer did to this user, and to no other user. All of us had the option to withdraw chips. The hacker just had above average amount deposited, and withdrew it in the most inappropriate manner.
And in case you think ChipMixer did not warn against that as a practice, here are their best practices from web archive:
Your deposit transaction is recorded on-chain. If you withdraw instantly especially large sum or unusual combination of chips at once then it can decrease your privacy. On the other hand if you keep some time between receiving chips and using private keys then your privacy grows with each ChipMixer deposit and withdrawal.
Plan your chip size
You know you will pay 3.5 mBTC every week. You mix 12 mBTC and receive 8 mBTC and 4 mBTC chip. If you withdraw them like this - two of your transactions will be linked. It is better to split 8 mBTC into two 4 mBTC chips to have them ready for each payment.