This doesn't prove the weapon's canonicity, and Miller reacting to it means nothing.
If there's whole bits of dialogue and conversation recognizing it as an actual piece of equipment they have, it's a thing they'd have, it exists. Especially in MGS, a verse that lives and dies on CODEC's and information being relegated to side talks, tapes, or notes. Hell, wait till you find out that Miller invented Dorito's and Mtn Dew in MGS canon.
And in the case of MGS, a lot of the cutscenes are dynamic, usually they'll have the equipment you have, in the cutscene, but regardless, even if that wasn't the case it wouldn't take away from the rest.
In fact, characters reacting to your actions happens almost all the time in those games (like killing the resistance members in MGS4 when you're supposed to follow them and Otacon getting pissed at you for doing so, Otacon telling you changing controller numbers won't work against Screaming Mantis, etc.) - that still doesn't prove that it actually happened.
Not exactly a good example, this isn't the same as explicitly noted contradiction it's "oh hey lol we made this, it exists, it works like this".
We know in the case of the following, that he didn't kill them, because it's directly said otherwise (And sometimes MGS decides to be weird and make all alternative paths canon simultaneously without explanation so even then), in cases like that yeah gameplay can be disregarded but just because it happens once or twice doesn't mean it invalidates the other two thousand times, something like that that isn't the same as CODEC's, text tapes and what not talking about or recognizing various weapons as being things that exist.
Or should we just toss all information from those because they aren't mentioned in a scripted cutscene instead? Like how The Boss was the first person in space, that's from a tape, should we ignore that because the Love Box has a tape but we don't see it in a cutscene?
Also that Screaming Mantis example actually
is canon it's recognized in Database, artbook and guide.
You're right, but if there's no other justification (no statements or Word of God of any kind to acknowledge it's existence) - then why should it still be considered canon/part of a character's standard abilities/equipment?
Because if nothing contradicts it then it is as it is?
Do we need a statement that Solid Snake can choke people out and break necks because he never did it in a cutscene? Even though it's a move he has in MGS1, MGS2, MGS4 and his predecessor has it as well in MGS3, MGSGZ, PW, Ops and so on. No, obviously it's something he can do, nothing says he can't.
Or do we need a statement that Big Boss can do a cool flip too because it's something he does in gameplay but doesn't do in the cutscene against Volgin? Obviously not, he can do it, nothing contradicts it, we have no reason to assume he can't, so he does.
And you literally just said Miller having dialogue acknowledging its existence isn't good enough (Database does too), you're being contradictory. If you expect everything to be given a huge lore dump, seen explicitly in a cutscene and so on, you're asking for to much, no game will ever do such a thing.
This doesn't refute my argument nor does this justify why Holy Water is part of Dante's Standard Equipment.
You're literally just saying it does exist without providing any evidence for it being as such.
Lad, it's on you to prove it
doesn't exist. Not on us to prove it does, I'm not saying it exists just because it does, I'm saying it exists because we see him pick it up,
use it, and we see it as a item that exists and so on. Now it's on you to prove that him doing that
never actually happened and doesn't exist. The burden of proof is on you in such a situation.
You're going to far in the opposite extreme, treating everything ever as canonical is bad as that's almost always not the case, but treating everything that isn't hyperfocused or given random attention to is just as bad, something that would fit your criteria would be like the DLC skins from MGS4, not something like the AK, Snake beating the shit out of mooks, doing a cool flip, or whatever.
To use a different verse, are we to pretend something like Mario's cloud power up in Galaxy 2 doesn't exist because he never used it in a cutscene? Or what about some of Samus' various beam and power ups she gathers but doesn't actually use in a cutscene such as the Beam Combo's from Super or X-Ray scope? Or what about Soma Cruz from Castlevania having every demon soul but only ever using 3 in a scripted event?
Obviously those things exist and are canon despite only being used in game.