I'm going to take one more crack at this.
Why does the type of Skilled Play of Moldvay Basic dungeoneering require a pre-prepped (mapped, keyed, stocked) dungeon while the type of Skilled Play of a mystery does not require a pre-prepped whodunnit matrix (who/where/with what/why)?
Its because of the multi-dimensional nature of dungeoneering vs the investigation + inference-exclusive nature of whodunnit-ing.
Consider the dimensional requirements of Skilled Play in a Moldvay Dungeon Crawl:
1) Exploration Turns are tightly encoded and unit based on multiple axes and they all matter (time, space, and how these integrate with the rest of the system).
2) The engine of Wandering Monsters every x Turns + Required Rest every y Turns means that Turns have to be tightly kept and players are making decision-points based on the integration of this stuff along with the integration of all of the other encoded bits of the system.
3) Loadout (which has multiple dimensions itself; HPs, Spells, Gear) has to be tightly kept.
4) Resource management has multiple dimensions to consider in both this moment, the next moment, and in the Crawl at large (including the question of "can we locate and fortify a location so we can make camp and recharge?").
5) Encumbrance + Hireling considerations (Porters have costs and they have to be protected) and this integrating with Gold for xp and the strategic decision to push-on vs withdrawal.
6) Decisions at the encounter level (parley, fight, evade, etc) are all integrated with everything above.
Simply put...if you're eliding, fudging, failing in your book-keeping/accounting, it impacts the actual signal of the Skilled Play of the delve. A person may not care about the "purity" of that signal of Skilled Play...but that doesn't mean that its not impacted by any of (a) not prepping, (b) eliding essential aspects of play which feeds back into the delve as a whole, (c) failing to book-keep all of the various moving (and integrated) parts, or (d) fudging rolls (either the GM or a player). Something as simple as removing Encumbrance (5) has a huge impact on play. Remove the Wandering Monster machinery (1) and Encumbrance? You're suddenly playing a different game.
Now you may like that game better, but the fact its fundamentally a different game cannot be questioned.
Now conversely, what is happening in a "whodunnit?"
* You're investigating framed scenes.
* You're putting together pieces of a puzzle.
* Finally, you're drawing an inference.
Those things do not require the acutely tracked, multi-dimensional, and deeply integrated operationalizing of play (including all of the mechanical resolution requirements) that is required in a Moldvay Delve. Those things require (a) a GM who can effectively frame provocative scenes that address the premise of the whodunnit, (b) players who can investigate/collate information, (c) and an inference that draws upon the coherence of the continual play loop of (a) + (b) until the puzzle is solved by a player.
I mean, you can do multiple continuous loops of Framed Scene > Investigate > Collate > Rinse/Repeat until Inference-based Conclusion and derive the same sort of Skilled Play in whodunnit-ing entirely in Unstructured Freeform without any mechanics and without a single prefabricated piece of the who, what, why, how puzzle (that stuff can be stitched together on the fly). Or you could have the who and why and have to stitch together the what and how. Or any 2 or 3 of that matrix and figure out/allow to emerge the last 1-2 pieces during the continuous loops.
Conversely, you fundamentally cannot do that in a Moldvay Dungeon Crawl. Its impossible. No human can keep all of deeply acute spatial/temporal information in their head and all of those multivariate interactions that occur in the course of a singular delve. The kind of Skilled Play that Moldvay Dungeon Crawls distills would be entirely lost.
The fact that a person might feel differently about the operationalizing of that whodunnit play doesn't mean that the litmus test for Skilled Play in that sort of play loop is lost. It just means they feel a certain way about it (its less real...its less substantial...less grounded perhaps). The fact that a GM/table might suck at operationalizing it, doesn't mean it cannot be done...because it trivially can.
But the other one. Its not feel. Its a binary of on/off. Are my individual delving decisions interfacing with all of the multi-dimensional and integrated properties outlined above such that the entirety of the delve was the product of Skilled Play? Yes, then Skilled Play. No? Something else.
EDIT - So what can No Myth Story Now fundamentally not do? It CANNOT operationalize the sort of high resolution dungeon delve in the vein of Moldvay Basic. Torchbearer, which is a Story Now game more akin to Blades in the Dark (but waaaaaaaay south of Blades in terms of resolution of setting), can do it (and do it awesomely), but it is certainly not No Myth. Its north of Dogs in the Vineyard Prep (which isn't quite No Myth but its not terribly north of it where you're preparing Towns which = pivotal NPCs and provocative, entangled situations which orbits around one or more Sins/PC Relationship and requires Paladin-ey intervention).