Right off the bat I want to says kudos, RGAZ, to you for your research. You took up my challenge and seem to have found evidence of Buick using hardtop sedan as a designation.
Now let's get back to stickling. First of all it is very hard (to me) to figure out on this website what was written by Buick, and what was written by the website. If you go into the owners manual and look at the page that shows "Models, wheelbases and lengths" the normal and unambiguous model designations are used, no mention of a hardtop coupe or sedan. Same thing if you look into the Chassis Service Manual. These seem to me what would be the, shall we say, official designations used by GM. Why would the hardtop sedan designation be used in ads, but not on the paperwork that the actual assembly and sales were tied to? The answer of course is that ads are meant to attract and sell, and not state actual, boring nomenclature. As I said before, when the rubber hits the road, and unambiguous designations are required, the body style names that are almost exclusively used are the ones in the owners guide and service manual. As a child we went to Boston, and while there I was asked if I would like a tonic. I said no as I thought that meant medicine! Come to find out that's what they called pop. So while some people in the northeast knew that tonic meant pop, I would venture to say the majority of the US wouldn't have. The people in Boston must have been aware of the name pop, as it was used extensively in nationwide advertising. So how do you reconcile the fact that to a small part of the population pop is tonic? To them they are correct. Do you go by majority rules? Do you go by definition? Do you go by region? I don't know, but we seem to have hit on all of these during this thread. These are all my opinions, and we all know what they say about opinions.
On a completely different note, I am into vintage tube and transistor radios. In the 1959 owners manual they show one of the coolest options in that arena. A transistor radio stored in the glove box. This radio was the heart of the car's radio, and was controlled by the in dash radio controls while in the car's holder. When taken out, it was operated by battery, and while out the in dash radio was inoperable. Buick called it Auto-Portable, Olds called it Trans-Portable, and Pontiac called it Sportable. Great stuff.