Is there a point to this analogy? Because I think the 5e "engine" works just fine for a sandbox, as do many other games.
The point--going back to the original use of the analogy--was that someone else had asserted that the rules are "just one element" of the game, and that it is incorrect, or even harmful, to say that those rules do not have primary significance.
If the rules of 5e did not "work just fine for a sandbox", would you not have to build your own? Would a 5e stripped of all travel rules, for example, not cause you to either look for a different system that
did have such rules, or to invent your own travel rules, rather than relying purely on, as you said originally, "non-prescriptive guidelines and advice"? I don't think "non-prescriptive guidelines and advice" would be adequate to carry a sandbox campaign.
Some rules have to be in place before you can focus your attention on guidelines and advice. Guidelines and advice can still be critical. Every game I play and enjoy has them. But they only operate when there's already rules to work with.
That you think the 5e engine works just fine isn't in question. You need a car that
can drive before it matters whether that car has any safety features or not--for exactly the same reason that "a structured framework that integrates adjudication tools, referee coaching, and worldbuilding techniques" cannot produce a sandbox campaign without there already being a foundation of rules for that "structured framework" to stand on.
The rules are the engine that makes the car go. That doesn't mean nothing else can ever possibly be important. But it is still most certainly the
first thing you need in order for a car to be more than a badly-made, overly-heavy Radio Flyer. First--primary. Hence, primacy. Rules happen first. Advice, coaching, best-practices, guidelines, etc. happen second. That doesn't make them bad, wrong, pointless, or any other negative words I'm sure folks will try to stuff in my mouth. It simply means they happen second in sequence.