Didn't you say Battle Spirit wasn't always active and they only turn it on like, sometimes during confrontations? When u say passive, I think people get the impression it is constantly on no matter what and everyone near them is always being affected by fear and paralysis hax.
Because Battle Spirit is weird.
You'd get cases like Rengoku,
where he just has his Battle Spirit on all the time. Gyomei sensed Kanao and Shinobu were sisters just based off of Battle Spirit (it's the novelization and I don't have the scan for it right now).
Akaza says everyone, even babies, produces Battle Spirit.
There's other stuff like Shinobu
not Fear Haxing the Spider Sister immediately (she snuck up on the Spider Sister). But then like, Fear Hax happens like 3 seconds later. Literally next panel after the following one Shinobu Fear Haxes. We have Zohakuten
looking in a direction and Fear Haxing everyone in the vicinity.
I think Battle Spirit is implied to just be a subconscious aura characters emits whenever they're adequately pissed off or in a fight. There's scenes of stuff like
Tanjiro in MT. Natagumo Arc Fear Haxing Inosuke despite the fact Tanjiro is mad over the fact Spider Mother killing people. You have 1000+ year old Muzan
throwing a tantrum and breaking everything within his vicinity simply because was mad. You got
Sanemi Fear Haxing Genya, and Tanjiro, who is hiding behind a corner, got hit with the BS too.
Then there is weird cases like Gyomei paralyzing the Hashira just by...
clapping his hands? Is this Fear Hax? It doesn't look like they're scared. Shocked, sure, but fearful? Nah. Something's holding them down, but if it was an LS thing literally everyone in the room would walk out of that no problem.
AND I STILL DON'T KNOW IF BATTLE SPIRIT AND FIGHTING SPIRIT ARE THE SAME THING.
Gotoge never explained it, so I'm stuck placing the pieces to the puzzle.
As for this match, Sans is getting Fear Haxed and paralyzed no question.
Muzan has his Battle Spirit on all the time. Even if Battle Spirit wasn't a passive thing, worst case scenario, Muzan looks at Sans funny and Sans instantly combusts