I think the sole Doylist reason for the line was to signal that Tony is about to do something disastrous that's going to cause more havoc instead of stopping war, which, well, is obvious for the genre wise anyway. And still doesn't explain on a Watsonian level why Tony Stark would suddenly quote Neville Chamberlain's candidate for "most famous political miscalculation of the 20th century summed up in one line".

Still liked the movie, though - on a middle between most and least favourite Marvelverse movies level, like I said - and am going to rewatch it today with local friends, so who knows what I'm going to get out of it the second time.