Yes it seems unlikely as when those go you get copper staining.I do have a heater, it is a Hayward with a nickel-copper exchange. It is only five years old. It seems unlikely that it is the source but I'm running out of places to look.
Mine is an older home as well and I have a mix of plumbing like that. I believe that one of my tubs must have some old galvanized pipe somewhere in line as sometimes I get some rust colored water out of just that particular fixture. If you are going to drain your pool again, try different brands of salt and CH increaser obviously…Mine is an older home, no PEX, copper and PVC instead. Plus, as you saw, the fill water from my spigot I use came back negative.