[hrunt]

Tsar Bomba was detonated in October 1961. That means that for any civilization to have seen it, that civilization must be within 63 light years of Earth. In addition, unlike the Pyramids, Tsar Bomba was a momentary flash, so if a civilization within that distance started looking today, they wouldn't see it. They probably wouldn't even see evidence of it, since here on Earth we can detect only traces that it specifically happened.

What the paper talks about is constantly visible evidence of human life. Civilizations up to 3000 light-years away can see evidence of the Pyramids because they were built 3000 years ago and have been visible from space ever since.