The answer they give isn't wrong but I'm suggesting that there is another answer to the question. Even though it is not directly stated the Hist are ultimately responsible for the death of the people in black marsh during the second era. I come to this conclusion based on everything we know of the Hist.
We speculate that the Hist are the cause of the Argonian's intelligence and there is a strong link between the Hist and Argonians. This is demonstrated several times in the elder scrolls series. The most prominent being in the novel when the Arganians are called back and later lead to their own destruction by Umbriel. We also know the Hist have a sap that when consumed can cause illusions and make people lose control of themselves but yet as long as its not bad sap it doesn't effect Arganians. Last, we know that the flue originated in Black Marsh and that it didn't effect Arganians.
This can be taken in many ways but when you take it all into account you see that the Hist have a tendency to manipulate the world around them in whatever way they please for their benefit and at the time eliminating the non-Arganians would have been to their liking. So in conclusion the Hist are the ones who created the flu.
This does not automatically make them responsible for the deaths it causes but I believe they are still responsible. For a better understanding I will compare it to a person who creates a knife. Where the person is the Hist and the knife the flu.
A person who creates a knife is not automatically responsible for who that knife kills but if a person creates a knife to kill a certain person knowingly and willingly he becomes just as responsible for the person who kills. To make my point further in this case the Hist used their knife, the flu, to intentionally kill people so they not only created it but they used it. This biological warfare is what makes the Hist responsible for each and every death that the flue caused. Sure the flu killed them but the Hist are ultimately responsible.