I interpreted it as being quieter than the previous model, and can't really see how "its new model was three times quieter" can be interpreted to refer to the i

I interpreted it as being quieter than the previous model, and can't really see how "its new model was three times quieter" can be interpreted to refer to the inside being quieter than the outside. So I suppose that there's room for confusion. As for why you should care about it being quieter from the outside: because noisy cars are extremely irritating to pedestrians and everyone else who isn't in a car (in the same way that people playing ridiculously loud music while driving in a car is irritating).