I admit, I was surprised to see such gunk on a cantilever. First thought was that the user didn't just 'dip' the stylus, but came into angle of contact with the cleaner surface that made with the stylus AND cantilever. In the case of a brush being used to clean, I would think the same thing. It is important as to how you Apply the brush. Then this stuff about magnetism comes up and it may be possible that static cling causes the dust to rise and contact the cantilever and stay there because of the buildup of cleaning agent.
More on the Onzow Zerodust
You may recall I posted on this back in December. Well here is more on the subject.