How can we tell we can trust anything in 40k since Tzeentch exist?

Tzeentch could easily twist the cognition or consciousness of human beings, he could potentially be the most powerful chaos gods, in a saying is that he only “lose on purpose” to continue the great game

But how could the people who know Tzeentch’s existence be at ease at all? What you plan? He might know, what you see or hear? He might manipulate it, you can’t even trust yourself at that point, that’s enough to drive you mad, and that madness fuel him as well

Are the people from 40k who know about Tzeentch suppose to live life as it is or actually know how to counter/“coexist” with Tzeentch?