In this section, Moldvay says:
"The DM should make the adventure seem as "real" to the players as possible. All should avoid getting stuck in long discussions about rules or procedures. The game should move along with humor, as well as excitement."
Here, Moldvay takes the time to reinforce what he's already expounded on in "That's not in the rules", which is to never disturb the flow of the game due to a question of rules. (And by making the adventure "real", I am confident he is talking about keeping the players' mindset out of the toolbox and firmly in the action).
It is certainly compelling to consider the idea of having the players know almost nothing but the most rudimentary rules of D&D. B/X is, keep in mind, an iteration of D&D for which players are extremely unlikely to have any sort of "players handbook". Even in 1E, where there is a players handbook, the lion's share of the nuts-and-bolts mechanics of the game is still largely confined to the DMG.
It begs the question as to exactly what sort of Pandora's Box was opened when rules were put, wholesale, into the hands of the players (extravagantly demonstrated by any 3E or later PHB) , and the DMG became less crunchy and more fluffy. Could Moldvay be any clearer about what he's talking about? Let me try it out myself:
Having Fun = Not worrying about the Rules
For all the in-depth dissection we like to do about the apparent disconnect between the OSR and the "contemporary" D&D community, Moldvay seems to have understood it all too clearly, even if it was 20+ years before it became an issue.
How did this principle get left by the wayside? Granted, a brief paragraph buried on page 60 of a "basic" 80's D&D manual is not exactly a trumpet fanfare announcing the Law of Good Gaming. But still, how exactly did the game's toolbox get passed from the craftsman to the homeowners?
But here we are, and Pandora's Box is already open.
How do we go back to that "age of innocence" when the majority of gamers you meet know full well how to build a "CR7 Defender" or the DC of a Climb check? Is it possible for us all to just forget what we know? To take a step back from our rules mastery of a system we pretty much know backwards and forwards and could probably, if time and motivation permitted, write out our own very complete version of the game from scratch?
Probably not. But we can, at least, try not to worry about the rules so much, or change things up on purpose once in a while.
Here's a homework assignment for all you DMs out there: change three rules you dislike on purpose (I'll let you decide whether you've told your players you've done so or not) before your next session, and try to make sure you use them. See if the players notice, how they react, and whether you think you can move them away from caring about it, simply because they are having too much fun to care.