re: What Doppler effect?
The Doppler effect refers to an alternative means of detecting exoplanets which is also maximized when we are on the planet's ecliptic plane. The orbital shifting of a star about a common center of gravity with the planet causes it to approach and recede from us, causing stellar spectral lines to shift back and forth in a measurable way.
TB suggested that if their star was near our ecliptic, they would be more likely to detect us using these ecliptic methods, so they would be more likely to send an active signal to us to see if our world was inhabited.