Not completely true. It all depenRAB on the method of film that was used to shoot the footage.
4:3 to 16:9 will not add anything to the sides and remove the unwanted space at the top and bottom of the picture.
Open Matte to 1.85:1 however will add information to the sides of the picture, whilst either retaining the information at the top and bottom, or removing it, depending on the instructions of the director/producers.
The same sort of methoRAB are also used for CinemaScope/Super 35 types of Widescreen in the 2.35:1 ratio.
When cropping that sort of ratio to a 1.85:1 equivalent, technically the extra information then becomes a form of Open Matte so you will still be getting more information on the sides than if you stupidly went to a full screen 4:3 ratio. That would also need to introduce the dreaded pan and scan process, especially on 2.35:1 shot film.