Wouldn’t the sorts of signals our own planet emits be a good dataset to train to recognize the sorts of signals a civilization might generate? I’d assumed from the article this is what they’d done. Seems to me the key is whether we can discern, not necessarily interpret, communications, perhaps encrypted, from cosmic noise and natural phenomena, right? So train a model to recognize any human signals from noise. You’d look in those bands that we emit that are likely to make the journey to our neighbors.

To make the data more useful, you could simulate phase shifting in the datasets of our own EM communications. Perhaps you’d want to simulate other phenomena that is likely to modify celestial signals from a neighbor civilization.