The author of this article is focusing only on SETI-based methods of finding extraterrestrial life. The most promising IMO, and one that I don't see talked about much, is the James Webb Telescope's ability to measure contents of the atmospheres of exoplanets. From what I understand O2 is only known to be created by metabolic processes of living organisms.
Also one thing that's curious to me is the math here assumes these millions of civilizations all exist now or have set up systems to broadcast these beams forever whether or not they go extinct. So if we make the following assumptions:
These civilizations only last a couple hundred years after they create these powerful directional radio wave beams (which if we use ourselves as an example seems to be a reasonable assumption)
One civilizations dies and another sequentially starts broadcasting radio waves (most favorable option)
It takes 2 billion years to create life
Galaxy is about 13 billion years old
There's been about 2M advanced civilizations in the Milky Way
That means there's a 3.6% chance there's a civilizations broadcasting right now. This goes up if some last longer than 200 years, and if some create machines that can broadcast either indefinitely or for a long time. Within 1 magnitude of relative certainty is pretty good I guess.