You arbitrarily ignore the far greater volume of evidence that in fact instructed me before ever I encountered the DMG. The textual fundamentalist waves "proof texts" at the orthodox traditionalist who is assuredly not about to chop off most of his understanding and "argue" from a feigned position of ignorance!In other words, the evidence you claim supports your position doesn't actually seem to do so.
You make much of "TSR". I do not very much care how the Bloom brothers or other corporate functionaries conceived of D&D, when it comes to the matter of how it was designed to be played.
As I recall, there were plenty of articles in The Strategic Review and The Dragon concerned with designing dungeons (and towns, and wilderness areas).
The most fundamental matter is not whether one's "dungeon" has but 2 levels or 20 (although vertical mobility is indeed important) -- but whether it is an RPGA-style railroad, a big and extremely non-linear and dynamic environment, or something in between.
Where it, and its wider context, lies on that spectrum has much to do with the probability that any given adventure is going to "clean it out".