>> So how could aliens organize so much of the universe to engage in broadcasting the same sort of signal?

Er, inter-galactic empire?

If we're going to start imagining aliens with hyper-advanced super-technology, why stop? We can imagine aliens that can do anything they like.

Then again, there is nothing that says some alien species can't have a lifespan of a few billion years, enough to make the trek from one galaxy to the next, with time to spare for lunch. Perhaps those bursts are their inter-galactic Skype, sending selfies back home: "Me standing in front of that hot new Supernova we just spotted!".

Like I say, we can imagine anything we like.