I would argue that all but one of those citations is in regards to internal player balance rather than Player vs Monster balance.
Yes. That's because Gygaxian AD&D is predicated on the assumption that
players choose what encounters their PCs confront.
This is spelled out in the advice to players in the PHB (about investigation, using detection magic, etc); and in the rules for evasion, which make it fairly straightforward for a party that takes sensible steps to avoid wandering monsters.
Wandering monsters, especially large groups of wanderers in the wilderness, threaten to undermine this paradigm.
This is why Gygax offers words of caution about wandering monsters very early in the DMG (p 9):
[T]he rules call for wandering monsters, but these can be not only irritating - if not deadly - but the appearance of such can actually spoil a game by interfering with an orderly expedition. You have set up an area full of clever tricks and traps, populated it with well thought-out creature complexes, given clues about it to pique players’ interest, and the group has worked hard to supply themselves with everything by way of information and equipment they will need to face and overcome the imagined perils. They are gathered together and eager to spend an enjoyable evening playing their favorite game, with the expectation of going to a new, strange area and doing their best to triumph. They are willing to accept the hazards of the dice, be it loss of items, wounding, insanity, disease, death, as long as the process is exciting. But lo!, everytime you throw the ”monster die” a wandering nasty is indicated, and the party’s strength is spent trying to fight their way into the area. Spells expended, battered and wounded, the characters trek back to their base. Expectations have been dashed, and probably interest too, by random chance. Rather than spoil such an otherwise enjoyable time, omit the wandering monsters indicated by the die. No, don’t allow the party to kill them easily or escape unnaturally, for that goes contrary to the major precepts of the game. Wandering monsters, however, are included for two reasons . . . If a party deserves to have these beasties inflicted upon them, that is another matter, but in the example above it is assumed that they are doing everything possible to travel quickly and quietly to their planned destination. If your work as a DM has been sufficient, the players will have all they can handle upon arrival, so let them get there, give them a chance.
I think that the advice about making numbers encountered appropriate to the strength of the encountering party is intended similarly.
Newer editions have both an implied and mechanical approach to encounter/adventure balance. Players tend to expect a very reasonable chance of defeating EVERY encounter they come across. Not so in AD&D, in my opinion of course!
Newer editions assume that the GM, rather than the players, choose what is encountered. (AD&D modules beging to make similar assumptions, I think, starting around 1983/84 with modules like Ravenloft and the Dragonlance series.)
But AD&D does assume that the players have a reasonable chance of dealing with EVERY encounter they come across - either by defeating monsters that the players have chosen to assault in the dungeon, and either by defeating or evading randomly encountered monsters.
(The absence of evasion mechanics from later editions is one mark in the change in expectation as to who chooses encounters.)