There are, unfortunatly several issues with that. The largest being the amount of time it takes us dominate our planet, achieve space travel, and expand enough to actually enounter The Eldar in time before Slannesh is born.
But there are other issues. Some of the biggest being that: The Lizardmen are highly likely to be seriously pissed that The Eldar are THE Strongest Empire in the Galaxy....but are busy fucking, doing drugs, and murdering each other in the streets to actually protect and govern the galaxy as The Old Ones would have wished, and that The Old Ones are as far as they know, Dead, and the Eldar have let their Mutual Creators Killers continue to survive instead of hunting down and exterminating every last Tomb World in Existence.
Not to mention they only did the bare minimum to suppres the orks instead of trying to exterminate THEM either.
Like....if the Eldar NEEDED to stop the War Against Heaven to get their numbers up and prepare for a future offensive, that would be one thing, but they've litterally done fuck all since the end of the war, and are content to descend into the darkest depths of Hedonism instead of crushing their enemies before they full awaken.
Mazdamundi is gunna PISSED once he learns about everything.
And THEN they might very well be solely responsible for fucking Slannesh into existence.
So Yeah. NOT A HAPPY SLANN.