PianoMan I never said that. Water that is 0TDS drifts to acidic after being purified, this water has too little alkalinity and is now corrosive. You are trying to strike a balance between water that will not scale, but will also not damage your boiler/espresso machine.
For example, RO machines with brass taps often have plastic lined tubing to prevent the metal being extracted into the water.
So if your water is 0TDS you add some minerals/alkalinity back in, this buffers the acidity. When you have some TDS you measure the alkalinity to get a handle on whether it will scale, or corrode, or be in the sweet spot.