I have small a R/O system hooked up from my water softener, I use it for my drinking water and making ice cubes, because straight hard water kinda tastes like shit. It's just a small unit that I can pull 1 gallon per hour. I tried using it for my plants when I used to grow, but it was a real pain in the ass filling a gallon milk jug and then waiting an hour to fill another gallon, especially when I needed 6-7 gallons at a time to feed/water my plants everyday. I remember the PH of my R/O water was always at 6.0 consistently.
The ppm of my R/O water was really low, maybe that what he meant instead? I remember my ppm's were barely measurable.