These old wives tales all started back in the day when equipment sellers needed gimmicky nonsense to try and get traditional guys to convert to WFP and spend, when you think about it we remove microscopic amounts of dissolved solids from the water which then is supposed to turn it into a supper dirt absorbent liquid, the joke is we all believed it back then simply because we didn't know any better.
I clean cladding and upvc fascias with tap water, they come up every bit as clean and shiny as they would with pure, the level of shine will depend on the condition of the upvc and there's nothing you can do with any type of water that will change that.