By the Emperor all this fucking heresy makes me want go purging.
But in the lore is stated, or used to be, that we created Nurgle in the dark ages, Khorne was personally made by Gengis Kan...
If this was retconned, the finally GA would have done a good retcon with the lore.
It was retconned. As it is right now, the Gods (minus Slaanesh) always existed, but were severely weakened and barely had any intellect or personality due to that. Then the Eldar fuck everything up (literally) and give birth to Slaanesh, who sends the other three into overdrive.
Damn space elves.
Except no.
It was like that (mostly) for always. Humanity didin't CREATE Nurgle or Khorne, especially Nurgle. The way lore said it, it was that Nurgle came into full power sometime around the Black Death, Khorne around Ghenghis Khan fun accomplishments and Tzzentch around the Renaissance. Correlation doesn't mean causation - Black Death was proably an effect of Nurgle coming into sentience, not what made him sentient. Tzzentch proably made people start considering change more and if anything, the Khorne birth was most related to Humanity considering that Ghenghis Khan became Doombreed, but it was more an recognition by Khorne to him rather than due to fact he personally created Khorne.
I mean, the fact that there were no fucking great huge warpshitstorms made during those times kinda gives you an idea that they got created in less reality-raping way than Slannesh.
The Cabal's vision - if it wasn't actually false and Chaos trickery - doesn't mean humanity created the Chaos Gods, just that the rapid and cataclysmic extinction of one of the most populous Chaos-supporting (Orks probably don't contribute a lot) race in the galaxy by the champion they've invested a lot of their own power into causes them to implode. That doesn't sound impossible to me - Horus at that point is so Warp-empowered he's like an Emperor Junior, ie, a god.
If, for some reason, you can't accept it, just interpret the vision as false. After all, Alpharius-Omegon did as they wanted and... it doesn't actually come to pass?
Mind you I didn't knew. That change was a good change then. Unlike what they did with Olanius, because nothing says noble sacrifice like a guy who knows he will come back up safe and sound within an hour or so...
What's funnier even is if the Cabal was right and Horus would eventually cause the destruction of the chaos gods if he won then Ollannius is the reason that 40k is so shit because without him around to be nobly fucked up by Horus the Emperor never would've mustered the balls to annihilate his son and would've just died without being interred in the Golden Throne, eventually leading to Horus' repentance and destruction of chaos.
good job bruh
No, fuck you, that's not what happened.
Cabals vision didin't actually came out as it should have - the fact that the thousands of years of war and stagnation wasn't exactly what was supposed as the "bad end" in Cabals vision was the fact that Emperor did not win. The "bad end" would be if Emperors victory but it would in the end cause humanity to get too bloated, populate whole Galaxy and ultimately cause maximum overdrive of warp or someshit, possibly by another brith of a Chaos God (Chaos God of Order?) which would proably fuck the Galaxy up for good. The "good end" was human extinction which starved warp, as the other races were either too weird for it (Orks), had measures against it (Eldar) or were far too small (Great Crusade removed a lot of Xeno races) to survive Cabals "mopping up" afterwards.
What actually happened was something in between - by not letting either Emperor or Horus actually win, Alpharius and Omegons schemes (and also of course, Ollanius) - wether it was on purpose or not - bought several thousand years of time for someone to come up with some other way to unfuck the whole situation.