Yeah Chaos and the Exatari have basically the relationship inverted. The Homeworld is in what amounts to warp backwaters/hinterlands and due to a lack of Narrative Traits, Chaos never really poked the Exatari early on in their evolution. This is further compounded by life on the Homeworld arising from Necrodermis based cellular structures which caused all souls to be tainted with C'tan energy which makes the souls disgusting to daemons. Then you have the fact that the Exatari just kill daemons fairly easily permanently due to their existence wounding species trait.
Atraxas is very confused on why humans are so terrified of daemons. Daemons have never really posed an issue to his species, so he lacks that intrinsic fear of Chaos that other species have. In short the Exatari are about the last species that Chaos would willingly go near, due to their biological makeup and the way their souls are pretty much toxic to warp lifeforms.