Those animals are pretty defenseless after they die and no one is immune to that.

You clearly don't get it. This is not about what they eat.

If you mix two chemicals, they react the same way. It doesn't matter if this happens inside a mouse or a human.

Their reaction to it might be different, which is what the article ponts out. But the gut biome still compares to what's inside a human, so if you introduce something like red meat, the measured reaction inside will be similar, even if it's not in their diet, like you shallowly point out.