I composed most of this at 4am my time I garantee its rambling
I am not familiar with that snip. It sounds like setting information, not game information.
It was a front and center to players in the player's handbook whatever you want to call it, and things like that set the stage for rewinding advancements for instance in what could be assumed accomplish-able by skills / without magic and this is the foundation for that DM adjudication combine that with mechanics as evidence - my fighter will get mechanical scrawny under - defined to poor advancement of 20 percentiles in something he is supposed to excel in (and no indicators really what having more skill really can actually do more at any level so you might as well call it a bust). Magic is supposed to be overwhelmingly more capable and it seems that way for non-combat at minimum.
To me that snippet defines an assumption and a goal, remember concept first design for 5e that is the paradigm.
4e explicitly said otherwise to that blurb many times and many places that the martial characters were able to accomplish analogous amounts of awesome via skill and discipline alone this is something they told both players and dms and it was pretty clear it was assumed and backed up mechanically by things like skill challenges and skill and utility powers (a great feature of 4e thrown away).
This blurb may be a small thing yes but it says up front you are supposed to aim low for non-magical ability and expectations for it;
What game assumptions are you talking about?
Bounded Accuracy is a design "feature" which impacts martial characters outside of combat far more than casters they do not get advancement in area or number of targets nor really any of the things spell advancement provides what they do not get reinforces the flavor text.
AND its even seems like there is a less obvious yoke on combat.
You cannot evoke the 1 man army of Chainmail when the game assumes you need to be challenged by zero levels and zero level enemies or even zero level challenges. I can find various hunter and monk and even a few fighter powers that can harm as many enemies as a 20th level 5e fighter and those appear at levels 1, 5, 7 etc... I think making Swarms and minions and solos and the like are an acknowledgement that enemies do need handled different based on the relative situation and that high level even in 4e had advancement for characters that wasnt obvious. Making a swarm of 100s soldiers and fighting the players in paragon could be handled either that way or with skill challenges. Aren't you the one who went through and changed virtually every monster in 5e?
So to me it really looks like the mechanics match the flavor text from that blurb ummm yay they succeeded.
To be clear, I was saying 4e played like 1e for us.
4e didnt play like it for me but it enabled certain things that 1e seemed to be aiming for (perhaps if my 1e DMs had extra awesome of their own I would be saying the same as you) and also elements 2e seemed aiming for but didnt follow through on - I loved the blurbs about inspirational sources for what a class was for instance from that edition.
More than "the latest edition doesnt seem to have me as the target" I felt each previous edition did things better than the last. (Someone asked for how long so yeh Bluebook - though recently going back investigating chainmail for inspiration)
5e is not better in my opinion it has bits that are better (backgrounds) but I do not trust that magic is even trying to remain balanced outside of low level combat (did they give up? Seems like it to me nobody plays high level and let the DM figure out non-combat balance seem not helpful).