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.