Sure, you can run a combat in a game of D&D without using the combat rules, but then you aren't, you know, using the rules. I suppose you're technically correct, but only in a somewhat trivial kind of way IMO. Your contention that the combat mechanics are somehow not rules is nonsensical. It's also not the case that opposed rolls going high is a mechanic of any kind in D&D as written, and it certainly isn't some sort of core foundational mechanic. Whether or not you think the combat rules are necessary has nothing to do with the fact there are combat rules, and that those rules are perhaps the foundational part of how D&D is played, next to the basic skills check mechanic. What you find necessary or interesting or think is needed doesn't change the game rules.
It is certainly true that the same basic d20 mechanic (more or less) is the randomizer at the heart of both general adjudication and combat adjudication. In order for your statement to be accurate you need to remove hit points, initiative, several key parts of various class abilities, the armour class system and a whole bunch of other things. It isn't a contentious or even debatable point that combat is the most detailed part of the mechanical chassis of D&D. You take all that out, which you certainly can, you can still play that RPG, but you aren't playing Dungeons and Dragons anymore. Perhaps that's where you were going? I'm not sure.
The issue here isn't context, it's that your contention about the nature of the rules is one that many people will disagree with for specific and rational reasons. Saying you don't need the D&D combat rules to adjudicate or resolve combat in D&D is nonsensical.
