Not sure I’d agree with that article. Moisture content is easy to measure with a $35 tester. PPM copper is not. Maybe Firestone knows this and has adjusted their sales pitch accordingly?
Our Audi A4 has a required brake fluid flush interval of 2 years, regardless of mileage in Audi’s maintenance schedule. My tester shows 4% moisture in the fluid right now after I did a partial flush 15 months ago so I am due for a full flush based on that alone. I don’t have the tools to do analysis of copper content so I have no idea what the PPM of copper in the fluid is. I’d bet it is lower than 200 PPM.
The Audi seems to collect moisture faster than my GM truck. Now, I do live where the humidity is high year 'round, so I’ll continue to use % moisture as my indicator.