Heh. Bear in mind it was the daemons of the Chaos Gods themselves who claimed that they're xenos creatures who just want to be left alone in the immaterium, when trying to sway Horus to their side. They're not exactly unbiased. The explanation of Jubal being infested by a xenos parasite is given because, even when his legions were faced with direct evidence of the powers existing in the warp, the Emperor was keen to continue shutting down any contrasting views to the Imperial Truth. Euphrati Keeler (prior to going all God Squad on everyone) challenges the idea that it was purely a xenos creature by photographing the screaming soul of Jubal while Samus has possessed him. A lot hinges on the definition of what makes a God - it's a bit of an arbitrary line in any case, as the Chaos Gods are still unimaginably vast and powerful entities whatever label you choose to attach to them. A rose by any other name would smell as sweet*. *Except a Nurgle rose. Don't sniff one if you value your nose and eyeballs.