Also, FTL technology existing would naturally abate the prospects of interstellar war under the Dark Forest theory, because it means FTL communication is possible; and factions that can communicate with each other quickly are far less likely to fight each other. This was, at least in the first book, a (iirc stated) reason why the Dark Forest theory exists.

Of note: It might not require the outrageous levels of technology you might expect to accelerate technology to the delta-v 3I/ATLAS is traveling at, simply because there are absolutely star systems near ours already traveling at a pretty large sun-relative delta-v. We get a ton of galaxy-relative velocity for free from our solar system; we just have to shoot the probe at slower solar systems. Putting (and surviving) biological life in there, however, is a different matter.

It would still require a stupendous amount of delta-v to slow down enough to be captured by the gravity well of the place you are trying to land though.

Also, the Dark Forest theory is based on the same game theory principles that said the US needed to nuke the USSR flat by the early 1950s, it should not be used without skepticism.

I don't know. We humans can communicate with one another quickly but war still exists, it just uses modern communications platforms for espionage, propaganda, attacking information infrastructure and controlling drone swarms.