The upper bound given in the paper is on emitters of one terawatt EIRP (10^12 Watts) and above. EIRP means an isotropic emitter would need to be this powerful to be detected. A beamed transmitter that happened to be pointing in our direction could be somewhat weaker in total power, and still be detectable.

It's important to realize that this is at least 100,000 times more powerful than our most powerful Earth-based narrowband emitters. So we would not detect ourselves in this search - not even close.

Moreover, we have no good reason to believe that an advanced species would have cause to radiate 1 TW into space. Our own terrestrial emissions are becoming weaker over time, not stronger, as we optimize for total system bandwidth in our communications.

So I wouldn't draw too many strong conclusions here about alien life. We are still very much searching for our lost keys directly under the lampposts.