Tri-Solaris knew only what their sect was aware of, not what everyone on Earth thought. Also - they were somehow aware of the dark forest, would be very naive to think others can't discover it on their own. They were clearly considering it a real threat, because they tried to stop him and reacted so quickly when the guy staring at the wall did the experimental star demolition.
> Humanity literally saw the probe pull ahead of the fleet and knew the probe was coming 50+ years before it arrived.
Wasn't it only seen because they were already looking there? I might misremember something.
> Further, they're able to suppress the Sun's broadcast capability.
Only after the girl is chosen as the person to hold MAD button, IIRC. At that point the decisions on invasion were made centuries ago. Counting on that happening would be crazy.
> Do you recall what light-speed capable ships do to the fabric of the universe?
Not really, I ignored most of that 3rd book technobabble because it was unphysical. They were destroying universe and making it 2d or something?
Anyway, why should it matter? Habitats are supposed to orbit somewhere pleasant, like any dumb rock would do, not move at light speed.
> No, the Tri-Solaris civilization still existed near the end of the universe. Their planet was destroyed, but they lived on as a space faring species.
My bad, don't remember everything from the book. Still - this only proves my point - it's possible, so they should have done that from the start. They were never in "existential danger", just lazy (but still somehow happy to spend lots of resources, effort, and risk to live on Earth).