Even there, Gygax's actual advice on this is along the ilnes of "follow the leads of play" rather than "design something to be sand-boxed" - see eg DMG p 87 ("After a few episodes of play, you and your camaign participants will be ready for expansion of the milieu"), p 112 (using ideas from other genres and other games to keep things interesting), though p 57-58 on other planes (emphasising creativity and imagination apparently for its own sake) doesn't emphasise the needs of play quite so much.Both Moldvay and Gygax are speaking to brand new DMs here, quite safe in the (accurate, in my experience) knowledge that once a DM gets some experience she'll be able to craft something of a world around the PCs either before or during play.
In other words, the campaign world will grow - but there is nothing to suggest that the campaign world is the point of play in its own right, as opposed to a deliberately crafted tool for letting the GM confront the players with challenges that will be fun and exciting. (James Wyatt got in trouble for advocating "the tyranny of fun", but on p 112 of his DMG Gygax reminds us that "ADVANCED DUNGEONS & DRAGONS is first and foremost a game, a pastime for fun and enjoyment.")
Fair enough. But I don't think that changes the fact that they're their for a reason, which is to support the role of the multi-level dungeon in supporting a certain sort of gameplay by incentivising players of high-level PCs to take their characters to the lower levels.While I know these guidelines exist I've never bothered following them.