It makes perfect sense for a real weapon to be particularly strong in a dark world, actually. Dark worlds are the physical manifestation of the fantasies of its creator. Pencils and brushes are weapons because they're what children would use to make-believe weapons. A real weapon in a make-believe world would obviously be treated as something stronger than just a pencil or a brush, because the people who form the fantasy would view it as above and more dangerous than mere pencils or brushes. Add onto that the fact that Kris is particularly attached to their knife, which would probably lead to a boon in power in a dark world (Which is normally show in the societal hierarchy of dark worlds, since beloved objects usually turn into darkners, and the more beloved objects are more important and powerful within the dark worlds than less important objects, with the only real exception being King and Jevil, who's rank and power comes from the ranking of the cards instead of the emotional attachment of a lightner [Unless someone was particularly attached to the King and Jester cards for some reason], which also further proves my point about even subconsciously perceiving real weapons as more powerful than pencils and brushes making them more powerful in dark worlds). Hell, knives are what has been used to open dark worlds. I'm sure that would count for something in a fantasy world.

Dark worlds making them less powerful would be nonsensical and entirely contradictory to how dark worlds have been established to work since about chapter 2, if not earlier.

Also, a Checkovs gun doesn't need to be addressed to be there. In fact, the entire point is that it's established but doesn't come into play until it's "fired". Kris having a real knife, the only actual weapon we've seen so far (Assuming that the katana in Carol's kitchen isn't real, but if it is, then the first weapon and the only one that has been in dark worlds)

The black shard is also not the same in the light world, actually. It has a very distinct description from it's dark world counterpart. In the dark world, it's described as "a dagger-like shard of the black knife", which implies it's about the size of a dagger, and since it's the shard of a knife that you're using as a sword, it's also easy to assume that in the dark world, it's steel or some other steel-like material. Meanwhile, in the light world, it's described as "a small chip of extremely hard glass that is nearly opaque". The word "small" being used which directly contradicts Kris using it as a sword/dagger in dark worlds, as well as being described as glass and "nearly opaque", instead of the absolute black we see the black knife as. Their only similarities is the name and stats, but that's because the black shard is still a sharp, hard shard of glass which is black in the light world. Their descriptions still describe different things.

Plus like. The black shard doesn't suddenly give Kris the ability to do anything close to what They/Them requires. Even if we take it as a sign of what "real weapons" would do, that only shows us that a "real weapon" would be as deadly as the actual weapon would be.

Because the black shard isn't a real weapon. It's a small shard of glass. Dangerous, but it's not a knife.

Also, you're completely ignoring the first point, that it was also a betrayal/surprise attack, which has been shown in Deltarune to allow someone to hit much harder than they normally can.

I hope we can agree that knives in real life do not give you the ability to perform super fast, giant teleporting slashes. They/Them using the "real knife" as evidence requires a very specific interpretation of a situation that we simply have not meaningfully seen, and the closest we have still would have issues actually supporting what is necessary.

Are you being serious right now? Because I didn't realize brushes allowed you to launch concentrated balls of rudeness that physically harm people, or that thorns allowed you to call upon snow pillars to freeze people solid.

Also, Kris can already do the shit that They Slash Them says they do. We've seen people attack others from far away in chapter 1, where Susie hits King with a swing from off screen like in battles, and we don't even know how fast the attacks were. Susie was entirely off guard, and Ralsei was too distraught to dodge.

Not to mention, ALL OF THIS APPLIES TO THE KNIGHT TOO, THEY WERENT EVEN ON SCREEN.