it will make it impossible to set regulations that will ban "bad" bitcoins
There are no "bad" Bitcoins.
I think that's his point.
They will still know that you own some bitcoin, but will also lose the ability to know exactly how much.
They do lose the ability to make any essential, valid conclusion. Unless the KYC-ed quantity of money is greater than the total value spent after the coinjoin, it's impossible for someone who sees you mixing to point over to your outputs, and therefore impossible to acknowledge that you own anything
at all.