Mixers are also businesses, like the exchanges.
Nothing stops them from an arrangement to not mix stolen coins.
Of course I don't know if this exists, nobody will confirm that.
When I've raised this possibility people have often come back saying mixers should not 'judge' which i think is total balls.
You can be a privacy believer and facilitator and not be a fan of enabling thieves to operate and escape. If I ran one any directly traceable stolen funds would be going straight back along with instructions as to where to ram their monies.