Well the reason for that is because salt on its on will not react with your cars metal, it needs water. So a heated garage that lets the ice melt and become water causes any salt to do it's damages, that's where the issue comes from. When it's below freezing, a lot of the water freezes and does not add anything to react to the salt. Realistically though, heated garages aren't an issue since they let the water drain away and evaporate faster. If your car sees a lot of snow, your car should be rust proofed, that should be Number 1. However some things to consider..
Water with salt in it won't freeze. That's why they use salt on the roads.
Water in a heated garage will evaporate, therefore leaving you with dried salt everywhere, as opposed to a mixture of salty slush on your under carriage.
Ideally, your car should have its undercoat sprayed with something to help protect it, that'll do a lot more than just heated vs. non-heated garages.
The cold will slow down chemical reactions. However, salt needs water to react. Having the car outside only prevents more water from getting on it, but it won't stop the existing salt and slush from reacting with metal.