A slipstream is the turbulence behind a force moving trough a medium, it's also the name of a short-lived science fiction movement focused on blurring the borders between science fiction and the mainstream. Neither definition have any direct connection to this story...
Magic disperses light in a peculiarly hexagonal way. You find traces of this pattern in the aura that appears whenever a unicorn works her craft. Even the most skilled of pony illusionists can't escape the fragmented edges that reveal her tricks to the trained eye.
As such, natural shapes are no problem to replicate magically. A unicorns thaumaturgical aura sparkles and sways in a way that mimics the shape of turbulent water and the rhythm of a gust of wind, but geometric shapes are more problematic. In the edges of squares, errant magic burn the air into angry sparks that can be concealed, but only with great difficulty and an intuitive understanding of gradients that only the most artistic of unicorns can claim to posses.
This is a long way of saying: There is no way in tartarus that any normal unicorn could store enough magic in the air to power a game centred entirely around shooting pixels.
Well, the story is very good. And, I have to say, a big improvement from your last "not writing with Lucefudu" story.
Still, there are loads upon loads of errors in it. Mostly confusions about "where/were", "was/were", "was/is" and so on. It, however, doesn't turn off the feeling you get from this story. While others do approach the "Celestia is withholding information from the ponies" line (parable?), yours is somewhat unique in the sense that it doesn't exactly say much about the whole thing.
And I personally do not know if this is a good thing or not... Mind you, it's not "things thrown in the wind and fuck you, reader", but it feels as if... if only it could be a bigger story (divided in chapters, maybe), it would be better.
I'm not gonna upvote or downvote this story because of that.
This story gets a Streptococcus agalactiae in the "how fast can a newborn die due to puerperal scepticemia" scale.
(That's a "pretty fast if you don't start pumping him with ampicilin and gentamicin" fast, in case you're wondering.)
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Thanks, the funny thing is that I have notice some "Luce"-ism creepy into the story, both a influence and as a reaction. For example, I've been using the word "prompting" more than I previously did.
I know!
That's what I get for self-editing. 
Our editor is busy on Dysphoria, but the real reason is honestly that the story was written from an "I'm gonna write a story where I do everything m'self!"-urge. And of course, if I do everything myself, I'm gonna make the usual mistakes I make on my own...
Yeah, I'm starting to feel like I should stop hatin' so much on Celestia in my stories, she can't help that Luna is better in every conceivable way.
Part of what I wanted to do with this story is to make it only as long as it absolutely needs to be. After all, if the worst complaint on a story is "there isn't enough of it" you're doing pretty good.
But maybe ending the story with a letter is a bit of a cop-out since it has enough material in it to be a story all on its own.
Well, I saw this being interesting, and tried it. And I must admit it's a good plunge to take! I love what you do with this, with how Twilight slowly piece everything, after all, it is indeed strange that while they have certain technology... they never did have the Industrial Revolution like ours. But in a way, I think that's also better, but I don't know.
The take on what is implied to be happening is really, really good though! I hope you are going to continue this, but well, this is already good in its own way.
Wait... it's over? ;-;