Why does contaminated brake fluid conduct electricity? Using a voltmeter to test for water contamination in brake fluid
Even pure water is somewhat ionized. If water is more conductive than pure brake fluid, the test is valid. Why don't you try this as an experiment? To sample(s) of brake fluid fresh from a new, unopened, can, add distilled water, drop by drop, stirring each time and look for a change in conductivity. Measure the proportions, and you can graph the effect of moisture on conductivity.
N.B. There are two types of automotive hydraulic brake fluid in common use: glycol-based and silicone-based. You should test the conductivity of each, and the effect of moisture on each.
BTW, unless so much metal was suspended in the fluid to make a thick slurry, that would not affect conductivity. It is possible some metals, such as aluminum, would react with the fluid and ionize, but those metals should not be part of the brake system.