This is fine - they're not pretending those impacts don't happen, they are just not what they're studying. They are asking "What does the fallout do to people some distance from the accident?"
The exposure people get early in the accident and very close to the reactors depends hugely on the nature of the accident. At Chernobyl, there were many firefighters within meters of an exposed critical core, resulting in a large toll from acute radiation sickness. At Fukushima, the cores ceased to be critical seconds after the quake and tens of minutes before the tsunami, and radiation was only released days later, so there was no acute radiation sickness.
By contrast, the effect of the fallout is much less dependent on the nature of the accident, just on how much radioactive material was released*. It can sensibly be studied without specifying details of how the accident happened.
* There is some dependence: the relative quantity of short lived isotopes such as Iodine-131 in the fallout depends somewhat on how long the radioactive material was contained prior to release.