I remember reading that report thoroughly at the time it was shared. I agree that the structure that ChipMixer used, and the similar structure that Whirlwind is now using, meant that they can't be broken in the same way as traditional mixers exactly for the reasons whirlwindmoney has given above. By allowing users to deposit different amounts to different addresses at different times, to combine and split these amounts freely, to do so over any period of time desired, and then to withdraw any amount of coins from their vouchers/notes, it becomes impossible to track inputs and outputs in the same way this report does. Of course users can still make mistakes such as combining mixed and unmixed UTXOs, but the service itself is not at fault in such cases.

My feeling would be that the fast option would potentially be breakable in the same way that every other mixer is, but notes would not be breakable in the same way that ChipMixer wasn't.

And of course if things get as far as blinded certificates, then it becomes provably impossible to link deposits and withdrawals.