If the energy required to send FRBs is equivalent to hundreds of sol-sized masses then it’s vanishingly unlikely that the phenomenon is non-natural. Any technologically advanced alien civilization willing to waste that amount of power to send a totally pointless signal across hundreds of millions of light-years would be, essentially, brain-dead and therefore incapable of constructing the necessary technology in the first place. There is no plausible reason for such a construction. We like to think angels visit us and aliens want to contact us, but that is only because we have tiny ape-brains and aren’t very good at dispassionate reasoning.