I can literally build a Jenga tower with standard size Jenga blocks using a forklift (we have little competitions every so often)(unfortunately I can't show pics or videos otherwise it'd be a security incident)
Of course it's assuming that's how advanced propulsion tech works. But it is useful to try to detect, just in case that's how it actually turns out to work, no?
And if we detect something interesting, like a potential warp bubble collapse, well, that also gives us a strong hint that it's possible, helping us to direct research in the right path.
Detecting techno-signatures of aliens would be super useful for us.
Sure, sure, you think looking for an intermix chamber of antimatter and matter that is not 1:1 is easy. What happens if they are simply leaking plasma from their nacelles? Or what if the ship is trying to steal your tech by luring an engineer onboard to repair “damaged” systems?
Or what if their warp drive uses an artificial singularity?
You seem to think that they are saying they can find any alien ship with any sort of technology. They aren't. They are saying that if an alien ship has this specific type of theorized warp drive technology and it fails, then we know how we would be able to detect that.