Question regarding chaos god presence in the lore in time

So I heard about this thing where if a chaos god exists, it must always have existed in the warp in the past, present, and future.

Now, I'm no warhammer guru. My understanding of warhammer lore mostly comes from wiki's and some youtube vids. But here's my thing, if a new lore clearly contradicts other established lore, I cannot in good faith believe it to be true even if the author or franchise says it is.

So right now, I have a major issue with this because the chaos gods quite clearly did not exist in the war in heavens. If they did not exist back then, how can GW say that they exist across all time? And for the dark god emperor... how can the chaos gods say that god emperor exists? wouldn't the emperor of man see the dark emperor in the warp and start battling each other?

So my question for the wh40k gurus... is there any lore that I'm missing that would justify the lore indicating that chaos god always existed in the warp despite my concern above? Or is it just the chaos god bragging and bs'ing about their existence with little substance?

What is the actual facts here?