In my 1953 Cyclopedia, Superior, in their ad, refers to their doors as "doors", but there are no examples in their ad of anything but the typical sliding freight car door. In a Youngstown ad, they refer to "sliding flush doors" (our plug doors).
There are no boxcars illustrated with plug doors--only sliding.
"Plug" doors do show up on reefer illustrations, though. They are referred to as "sliding doors", "sliding side doors", and "Youngstown sliding doors".
I think that, at the time, "everyone" knew that all doors on boxcars were sliding, so they were just "doors". On reefers, old style doors would, I guess, just have been called "doors" or "reefer doors" until the newfangled style showed up. Since reefer doors normally didn't slide (although boxcar doors ALWAYS slid at the time), it probably made sense to call the new style "sliding". 'Cause they did. And regular reefer doors didn't. When the new doors were put on boxcars, calling them "sliding" wouldn't work any more. Certainly "sliding flush doors" is properly descriptive, but it's awfully long. So, I s'pose they needed a new name.