Yukari can see the fourth wall. #CONFIRMED
also Momiji can bite the fourth wall. the implications of that will not be explored
There was a train of thought I hopped on a while back when I found out about this dialogue. I think this is in reference to one of the spell cards she does, Chewing Satisfaction, but specifically as pulled from Impossible Spell Card? As in, the game where everyone is actively trying to attack and maim Seija.
To reiterate: This is the game in which the conceit is that all other characters have been pushed to their emotional limits by another person who By Nature has literally no choice but to follow a lifestyle of evil, and are thus seeking to break the normal Spellcard Rules to try and actively eliminate her either by capture or destruction.
And yet Momiji is described here as if she's not even attacking her opponent, instead 'biting at some observer from another world'.
Momiji's ability is having really, really good eyesight. Usually they just shorthand it to seeing 1000 ri ahead, but I've sometimes seen her given a title more akin to 'Eyes That Perceive Reality'? Would love to have an official citation on that for you but I'm having trouble digging it up right now. The point is, she can see things other people can't, and Yukari and Sumireko are clearly extrapolating from all this that she's Capable of at least understanding that there is a player involved here well enough to try launching an attack against you directly. Why would she be bothering to do this in the game where every single other spell card is running on the logic of 'we need to make our danmaku unbeatable so we can defeat Seija Kijin, the Amanojaku'?
Well I mean. Think about what it looks like from Momiji's perspective.
She spends all her time on Youkai mountain, and has heard in passing that a wanted outlaw, Gensokyo's Most Marked For Death Twink (since Tsukasa wasn't introduced yet), might be passing through.
It is literally her job to keep people off the mountain. She probably has no use for the reward, lord knows it would be scooped up by her superiors and the crow who reports on it best would get all the credit anyway. So she's going to stand post and do what she always does.
Seija arrives, and naturally is fighting her way up the mountain to escape the literal crowd of people trying to headhunt her, and she's doing it really well. Like, insanely well. Like, that is a cloud of people swarming her, and somehow she is still dodging everything. Canonically she probably has not taken a hit yet. Yes, she has her weird cheaty artifacts, but this is the combined might of everyone in Gensokyo against some lady who wasn't even the final boss of her own incident. It stopped making sense at some point.
Her opportunity strikes. Momiji gets out there.
And the first thing she sees, is the thing that is making this happen.
Some strange entity she can faintly make out that doesn't seem to exist in her reality is puppeteering every one of Seija's moves. Every discrete motion she performs to twist around bullets, all those big sweeping illogical dodges, all those clutch moments using her weird magic relics? They map 1:1 with the strange movements of this Thing she can Just Barely See, that nobody else seems to be reacting to.
And, from her perspective... suddenly, it all just might make sense. How many people are in Gensokyo who have had bad days but come around eventually? How has almost everyone else been shot at into submission and become at least a relatively pliable person to talk to? And yet Seija, specifically, is supposedly the only one who has become so bad that she has to be dealt with by everyone without spell card rules? The one person who, given a second chance, kept choosing to be evil on purpose?
So she thinks to herself,
What if it's not all Seija's fault?
What if they're all wrong about her 'choosing' to be evil?
What if Seija Kijin is the victim of something bigger than this, and she's apparently the only one who can tell? That apparently she might be the only person who could save this girl?
What if Momiji Inubashiri was the one person in all of Gensokyo who really, truly just wanted to believe that there could be someone worth saving in Seija Kijin, even after everything she's done?
So she fires her bullets. And she aims at something else.