I thought this was interesting because it was widely believed that "ordinary" radio signals could not be detected from even nearby stars, only intense beams *specifically* targeted at us as a form of communication.

Reading the paper, the answer is that this was true "until now". Even the Mileura Wide-Field Array (the focus of the discussion in the paper) can only detect Earth-civilisation strength signals from the nearest 1000-odd stars, after integrating for a whole month. Of that 1000-odd stars, there are many that would not be likely candidates for "life as we know it" but a determined survey of all 1000 would take ~100 years with this array.

Future planned systems will be more sensitive still, and reach further out.

The Authors are careful to point out that SETI for it's own sake is a problematical endeavour, and they pin their hopes on a serendipitous discovery of an ET signal during other observing programs.

As usual, the difficulty will be proving that any received signal is NOT leakage from terrestrial sources (either on-ground or in-orbit). Calls for a large receiver array on the quiet far-side of the Moon will, of course, address this particular issue.

[Severely ironic smile, since this invites another "moonbat vs marsbat" debate. Please note that Moderators will hose down any unseemly discussion forthwith!]