I don't think I understand what you are saying. It sounds like that leaves no room for GMs to write their own adventures, or make up their own monsters.
Well, in Moldvay Basic an "adventure" is a dungeon, and the GM is expected to write up those. There is a whole chapter that sets out very good guidelines on how to do it.
When it comes to wandering monsters, the GM might write up their own wandering monster table, but I would expect them to adhere closely to the precedent set by the published tables, in so far as these establish what is appropriate difficulty for a given dungeon level.
I'm also confused about how you are defining "skilled" play. If I ignore the wandering monster table and instead of 2-8 Hobgoblins I pick 20, and instead of "neutral" I pick "hostile", there is still plenty of opportunity (and more urgency!) to demonstrate skilled play.
I don't know if you have access to a copy of Gygax's PHB. If you do, read the section, which is at the end just before the Appendices, called Successful Adventures. That contains advice to players, but from it you can also see very clearly how Gygax expects the game to be played.
He particularly emphasises planning, with multiple layers: planning load-out (gear, magic and spells) with a close eye on the goal of a dungeon expedition; gathering information, so that goals can be rationally formed and pursued (this connects to the "exploration" aspect of play, mapping, listening at doors, and all those other similar components of classic D&D); not being distracted from a goal once it is set and the expedition has set off; as part of the preceding, avoiding wandering monsters or otherwise dealing with them as effectively as possible.
If the GM is going to make decisions about monster placement, numbers, reactions and the like ex tempore, in ways that render collection of information and planning impractical or even impossible, the game has changed very significantly from what Gygax set out. (And what Moldvay's edition is also written to support.)
I personally don't play Gygaxian D&D very often, and am not its biggest fan - I find it a bit tedious, as I lack the patience it requires to be done well (both by the GM and the players) - but the approach is a clear one, and Gygax's PHB and (at least parts of) his DMG set out a clear framework for doing it; and Moldvay Basic is an even cleaner set of rules for that sort of game.
Generally I play games where the GM is expected to make decisions about encounters with NPCs ex tempore. But those are not really "skilled play" games, at least in the Gygaxian sense. And there are still a whole host of principles that govern how those encounters are framed, although they are different from the combination of map-and-key + wandering monsters that govern the Gygaxian game.