That makes no sense. I'm talking about classic D&D, not the MCU.
You are drawing an arbitrary line through the product line and saying "this product is the true identity of the implied setting, but THOSE products are not."
Every time someone points to other products in the same line, you are discounting them because they're not the "true expression," which isn't a characterization anyone, most especially TSR, would have ever recognized.
Mystara isn't just the map in back of the Isle of Dread as much as you -- or I! -- may prefer it to be. It's also all of those later additions that included straight-up joke settings, a few problematic adventures and gazetteers, and the later boxed sets (even the 2E ones, sorry, everyone!).
The Forgotten Realms isn't just the gray box. It's not even Ed Greenwood's original articles in Dragon. It's everything that's come since as well, including the movie.
And D&D as a whole isn't any one point on a line, but the line itself. OD&D includes all of the supplemental booklets, which both expand and fundamentally change the game. AD&D isn't just the three core books, but garbage like Gargoyle and the Forest Oracle and the completely superfluous books like Greyhawk Adventures, as much as I may hate it.
You can choose to exclude stuff from play
at your table, but when characterizing any of these lines, you can't say that just a portion of them is the "real" expression. It's all of those things in the line, for better or worse.
Instead of dealing in absolutes, dial it back from "D&D is X" to "I prefer D&D to be X, meaning using only the books up to Y point." Granted, that would lop off five pages of this thread, but maybe that's OK.