Overall, this method is fine IMO for trading kyc for non-kyc coins. The kyc exchange still has record that you bought tho.
This method is less acceptable for "washing" tainted UTXOs. If law enforcement sees the utxo enter a fixed float address, all they have to do is subpoena fixed float for details on that swap. Then, they have a lead on the next hop in the trail. If you're using a custodial LN wallet to receive your fixed float LN swap, then they can also be subpoena for additional details and potentially find the next hop. Even muun which is non-custodial can be subpoena because a fixed float swap to LN muun wallet will have paid muun's public LN nodes and there is a record in muuns database which links that swap to a UTXO in your muun wallet.