What was your response when that suggestion was made?
If a dealership said that to me, I'd be asking for a written explanation of how washing a car causes corrosion, which I'd be passing on to my solicitor. If they refused, I'd get out my mobile, tell them I'm taking a video and ask them again why the chrome has corroded and I'd have no hesitation in telling them it's my belief that 1: they are lying 2: they are not in anyway qualified to say how a piece of trim has corroded.
That's such a cop out it's untrue. Are they suggesting you've washed your car with caustic soda? Why not ask them for two copies of their conclusion so you can also send one to Autoglym (or whatever cleaner you use) with a cover note: My SEAT dealer says your product is corroding my car. Come to think of it, ask them for five copies - tell them you'll also be sending one to WhatCar! and Which! saying my SEAT dealer says XXX's wash n' wax has corroded the trim on my SEAT. And you'll be sending the last copy to the institute of mechanical engineers because you've found an expert in metallurgy who can diagnose faults in metal without the use of a microscope.
Or you just call the person an ignorant prick and ask for the dealer principal.