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I've always imagined that the Tyranids vs Chaos thing is largely decided by scale; two opposing forces ranged against one another of variable strength. Larger numbers of Tyranids generates a stronger Shadow, and the stronger the Shadow the more resistant they are to various Warp effects.

Thhere's a 13th Black Crusade section on an Imperial world set upon by Typhus, only to in turn be invaded by the Tyranids and the Tyranids there were immune to the diseases Typhus unleashed.

It goes both ways, and is very hard to measure.