There is some discussion at https://space.stackexchange.com/questions/17765/from-how-far-away-could-our-seti-searches-detect-and-recognize-our-technological .
Apparently, we do not yet have big enough radio telescopes to detect "leaking" Earth-like radio signals from another planet, so SETI hoped to discover an intensional signal. For example, many early SETI efforts searched for a radio signal around the "water hole" frequency, hoping that would be a Schelling point for trying to communicate with aliens.
But the great filter problem is not just about Earth-like civilizations. Fermi thought that technological civilizations should quickly become big enough to settle entire galaxies, so then the "great filter" is whatever prevents that—just inverse-square radio signals is not an explanation for that.