Most score you on style as well, not just efficiency.

Right, but the style has point values assigned to you. If they're unchanging, there is a way that will always work best, every time. At a high level (correct me if I'm wrong, as I'm somewhat new to this genre), rewarding style is similar to rewarding variety, juggles, and getting multiple enemies in the same attack. If you go down the checklist of your arsenal, you can always hit the variety. If you know exactly how the enemies behave, you can reliably get multiple enemies in the same aerial combo that the scoring system rewards most. The same actions give you the same output, and one of those score values will be the highest out of all other possible options. One set of actions will reliably always handle the same mob if it's deterministic.

Hmm… how does that work? I hit my opponent, they take damage, no Xcom bullshit. I don’t see any RNG-like behavior in this interaction.

That's just damage. The rest of the fighting game is rock paper scissors. A beats B beats C beats A. At round start, what button do you press? There's always some option that beats your option, and that's before we've even calculated the resulting damage. Some of what they're doing is responding to what you've been doing, but the rest of what they're doing is trying to be unpredictable; AKA random. (And that's before we even talk about characters like Faust.)

Keyword is enjoy. I don’t see myself replaying DMC5 for as long as I’ve been playing some of my favorite games because I enjoy it less.

That's interesting. As I said, I'm somewhat new to this genre. The short version is that Hi-Fi Rush got me interested in checking out all of the DMC games (minus the reboot), and 5 ended up being my favorite of that series (but still not as good as Hi-Fi Rush).