as per my experience all staff in all dealerships is just kids on daily wages with some basic training and basic technical diploma from any technical institute. i have vetted the same information from a technician with detailed interview.
you are right that reading the manual is important and following it accordingly is important too.
another important thing to consider that the glory is almost new car and it actually came with the right amount of coolant and water mix in its system from factory, local road side liqui moly and amsoil and kendal dealers have created a myth that these products are superior to any other product with proper certification rating mentioned like zic, kixx or atlas etc. i say its a way of looting people by selling them the same coolant at a much higher price.
anyways as jake pointed out above you need to be bit kinder. we all are not experts like u r. so conveying the same message in a humble manner helps and gets through better.