Here's the way I had it explained to me.
The pH Perfect Nutrients are guaranteed to work with RO water. They will work with MOST but not all water sources that aren't hard. So if your ppm is under 200 or maybe 300ppm, it will probably work as advertised. However it's impossible to really say that for sure without knowing how many ppms or what ions because there are potential dissolved solids that would screw it all up.
I've also been told that even if the pH goes outside the target range the nutrients are so heavily chelated you won't get lock-outs but I haven't confirmed that and this wasn't something AN said to me in any official way. It was more of a "this is what I heard but I don't know if it's true so don't quote me on it" kind of thing.
I use RO so it works flawlessly for me. But if I were having that kind of problem and using municipal water I'd definitely consider letting the pH drift a little (nothing too crazy, but farther than I'd usually permit) to see if the plants start to show sign of pH stress.
It's possible the rumor is true and you can grow with the pH Perfect nutes even way outside the usual target pH range. It'd be worth knowing.