If we can say this of 3e, then we absolutely should say the same of 4e. 4e wasn't a treadmill, and anyone saying it was a treadmill simply misunderstood how to run it, even though the books were quite clear about these things. (E.g. explicit instructions NOT to use only encounters tailored to the party's level, but a mix of encounters across a fairly broad range of levels, e.g. anywhere between level-4 and level+4, favoring high variety.)
I have no personal knowledge of 4e, so I can't really agree or disagree with you on if it's true of 4e. I did hear more than once in 4e discussions that DCs were supposed to scale with the group, and we all know that since I heard it on the internet, it must be true!!!
If this is what people have meant by "rulings not rules," they've done an absolutely terrible job of explaining it for literally a decade at this point. This doesn't, in the slightest, look like "rulings not rules" to me. It looks like treating the rules as an extant baseline, and then building new things on top of them. It's not that you're treating the rules as mere suggestions with no validity. Instead, you look to them for grounding, and build upon them with additions where you need such, only overriding or overwriting them when a serious issue comes up. That's a hell of a lot more cautious than any presentation of the "rulings not rules" concept I've been presented with.
I have two thoughts on this.
First, I think that you are partially correct when you say that it's not about treating the rules as suggestions with no validity. The "rules" are valid. However, it's also clear that they are also just suggested rules. I went through the DM last year I think it was and listed a huge number of times it calls the rules guidelines(suggestions). I'll list just a couple now.
"Chapter 3, "Creating Adventures," provides guidelines for designing combat encounters using experience points."
"AS THE DUNGEON MASTER, YOU AREN'T LIMITED by the rules in the Player 's Handbook, the guidelines in this book, or the selection of monsters in the Monster Manual. You can let your imagination run wild."
And of course at least a half dozen times spread throughout the DMG where it says that the rules serve the DM, not the other way around. All of that indicates to me that the rules are suggestions for the DM to build on or override when he feels it should happen, not just when some serious issue comes up. They are encouraging house rules.
Second, 5e seems to be deliberately designed to force rulings. There are far too many "rules" or "guidelines" that are written vaguely and/or with common situations involving that rule that are just plain missing.
The designers are forcing rulings over rules in this edition.