The key word being estimate. In most cases where flu is attributed as the cause of death, it's usually just a guess. Rarely if ever is a thorough autopsy conducted to determine, definitively, that the person in question died from the flu virus. Because in the overwhelming majority of such cases, the person is already quite old and feeble - basically, they were already at death's door. So whether it was a flu virus or some other opportunistic infection that killed them is largely moot at that point.