ok elaboration for people who have never heard of the deltarune omphalos theory (everyone, I made it up just now):
so a main form of suspending one's disbelief (for videogames specifically) is assuming that the game isn't everything. there are areas that characters can access and talk about that you can't, because developers have lives and can't model whole planets. there are events that occurred that aren't coded into the game, because developers can't model the entire history of a universe. In Deltarune this assumption comes mostly in the form of the Device theory (ie. a character in Undertale is connecting us to the universe of Deltarune through a device, shown in the naming schemes of many menu elements). The base assumption is that the universe described is real, we just get an imperfect altered glimpse of it.
Deltarune loves to play with this. The list of things that have been confirmed to be diegetic includes:
the config menu (and presumably the rest of it)
the top-down/4th wall perspective
the fact that Deltarune is played on a screen (and sometimes a device with a microphone)
It's also kind of an extreme version of the limited/blocked off area thing. Unlike the sprawling and detailed imaginary worlds, the accessible 'real world' is really quite small and bland, despite what we've been told about the forest and Asriel's college. One road in, one road out, and it's currently blocked off (kind of ineffectively) for some reason. (the reason is of course we can't leave Hometown because nothing is outside of Hometown, but suspension of disbelief etc.)
the real life omphalos theory basically exists to reconcile the fact that the bible says the world was created ~6,000 yrs ago, but we have conclusive proof the earth and the rest of the universe have existed for longer than that (light from distant galaxies, fossil records, etc). The theory goes that the universe *was* created 6,000 odd years ago, but in such as state that it was as if it had existed for billions of years, because if you put a bunch of baby animals and plant seedlings on a barren wasteland things won't turn out too well. It's named after the navels that Adam and Eve presumably had despite never being born ('omphalos' ['ohm fal uhs'] means navel in greek). A variation of this is Last Thursdayism, where instead of ~6,000 yrs ago it was last thursday the world was created, and everything just appears as if it has existed for a long time before that.
See again Noelle's Weird Route speech:
So, why don't we do it?
Why don't we do…
"Something crazy?"
Tell me.
Tell me to grow wings…
And let's fly out of this stupid town!
To somewhere no one has ever been before!
The other side of the lake.
We always wanted to see what was there, didn't we?
The four of us, when we were kids…
Kris, let's go there.
If we treat Deltarune like a real universe, that makes no sense. Why would no one have ever been on the other side of the lake? It's right there! Has no fishing trip ever been planned, no scenic drive ever been driven...?
However, when we treat Deltarune like the game it is, when we stop suspending our disbelief, it makes more sense. No one has been over there because it doesn't exist in the game files. Then, we apply the same logic to college, to Asriel, to Dess, to the rest of the universe. That's not in the files, it's not real, characters only act like they're real because that's how they were created. That's sort of the point of the game, right? Trying to escape the story, but there's nothing there. That's what happened to Dess, that's what Spamton tried to achieve, that's what Kris and Noelle are working towards in the Weird Route.
So much more of the game makes sense when you realize that diegetically, within the game universe, Deltarune is a game (that sounds stupid bear with me)
"why can RK see the bulletbox" because he's just as real as it is "why can flowery change the menus" because he's just as real as they are "why can HERO_SWORD exit its fictional world within the fictional world within the fictional world" because it's all just as real.
So WR Kris and Noelle, they're trying to go somewhere new, escape the 'snowglobe' that is Hometown, where nothing ever changes because nothing ever was. Whether 'the other side of the lake' exists (or will have always existed ?) we don't know yet. but gosh am i excited to find out...