There seems to be a misconception about the term “plate mail” from TSR-era D&D among armor enthusiasts dealing with spread of D&D jargon as well as among some gamers.
D&D plate mail armor is not plate armor. In D&D, “plate mail” refers to mail armor augmented by some pieces of plate armor.
In D&D, actual plate armor—distinct from plate mail armor—is called “suit armor”. (D&D Master Players’ Book p. 15) In AD&D, plate armor is called “field plate armor” or “full plate armor”. (AD&D Dungeon Masters Guide p. 27; Unearthed Arcana pp. 75–76)
Yes, “plate mail” is an unfortunate term. (I’ve been using “mail and plates” until I find a better term.) It should be considered game jargon rather than a general term or historical term. And it should not be considered a synonym for “plate armor”.
(You may notice that plate armor, likewise, is typically augmented by some bits of mail.)
Note that—unlike “plate mail”—the use of “banded mail”, “ring mail”, “scale mail”, or “splint mail” in D&D is not similarly justified. It would be better to drop the word “mail” from these terms.
For what it’s worth, banded and ring armors might never have existed.
While “chain mail” has come into common use, in medieval usage it was called simply “mail”.