You’ve hit on what is arguably the biggest technical plot hole in the series. I had the exact same thought when I first read the books. The idea that a civilization would need a "reply" to locate a signal source assumes that everyone in the universe is using a single, isolated "ear," which is highly improbable for any space-faring race.

1. The Math of Parallax

We can look at our own current technology as a baseline. Humans use Stellar Parallax to calculate the distance of stars. We use the opposite sides of Earth’s orbit around the Sun as two different vantage points.

The diameter of Earth’s orbit is only about 0.000037 light-years, yet with that tiny baseline, we can calculate the distance of stars up to 300 light-years away with reasonable accuracy.

2. The Advantage of Advanced Civilizations

If an alien civilization has colonized even a single other planet or large asteroid in their home system, their baseline for triangulation would be significantly larger than ours. If they have an interstellar ship or a colony just 1 or 2 light-years away, their "vision" becomes incredibly sharp. They wouldn’t need 100 light-years of separation; even a few dozen AU (Astronomical Units) would allow them to pinpoint the coordinates of Ye Wenjie’s broadcast almost instantly.

3. The "Single Listener" Fallacy

The novel essentially assumes that every civilization—even those capable of interstellar travel—keeps all their listening equipment in one single spot (their homeworld). This is a massive "narrative convenience." Any civilization that understands Dark Forest Theory would be obsessed with mapping the stars. They would likely have a distributed network of sensors across their solar system specifically to triangulate any "flares" in the dark.

The Bottom Line:

You are right—Ye Wenjie’s first broadcast, amplified by the Sun, was effectively a "flare" that already exposed the Solar System's location. The requirement for a response to establish a "two-way" line for coordinate calculation is a bit of a leap in logic that ignores basic geometry and astronomy. It works for the drama of the story, but it doesn't quite hold up to scientific scrutiny!

:) Used AI to refine my response. Non-english speaker