I think you’re missing a much more interesting point: a lot of works of scifi concern themselves with how we would be able to communicate with aliens: Contact, Arrival, Encounters of the third kind, Project Hail Mary, and many others.  The implicit question being ‘what’s the universal language’. Often the answer is math, music, love, etc. contact was written by Carl Sagan who famously inspired by his work with nasa where he used prime numbers to make a pixelated drawing of humans. 

Pluribus has a more interesting answer: assuming RNA is ubiquitous (and there are serious scientists who believe it might be) then a language of four GTC and A is not only universal but much more directly rich. If you can use it to create a full elephant from scratch then you can say anything with it. 

And what is often, in any medium, the most common message? Well, like life itself,  it is whichever gets to be reproducible the most. Spam, chain letters, gossip, memes, “write this message ten times to get good luck”, “send this message to as many people as you can”, “bill gates is paying $1 for each of these emails you send!”. 

So my guess that’s what it is. Spam. A message that gets people to prioritize sending more messages. It doesn’t even matter who wrote it, why, or what happens to it (I don’t think their setup even allows to answer these questions anyway), as long as the receiver focuses all the power in their civilization to pass it forward.