I think you misunderstand the point of gabipurcaru's analogy. nmc's confusion was about time, not energy dispersion. That is, nmc indicated that we would somehow be safe because the explosion happened 12 million years ago. But gabipurcaru pointed out how long ago something happened is irrelevant if the energy that reaches you is high enough.

You are correct that we are safe because of energy dispersion - or, put another way, because it is 12 million light-years away. But nmc thought we were safe because it was 12 millions years ago, and gabipurcaru tried to show by analogy why that is not true.