For what it's worth, the Guardian logs actually don't go into enough detail to say if their conflict was a purely defensive one. Like seriously, it's something like three sentences of vagueness. It is described as in "Guardian space" but has also been compared, sometimes in the same sentence, to the events in "human space" - which at the time, was the Pleiades. The details of both sides actions in the conflict are also glossed over, not to mention what was even said in the talks...Reading through the recent comments highlights one problem that has been bugging me:
Why do the Thargoids react so aggressively to Guardians technology, when the war with them was not only so long ago but not, according to the Guardians lore we have seen, on the scale of our own engagements with the Thargoids?
We have used weapons of mass destruction against the Thargoids twice, once successfully. For the last few decades in particular we have expanded our territory aggressively and into regions the Thargoids have claimed (Pleiades, Witch Head, Coal Sack, California Nebula), whereas the Guardians have no sites in these areas (save for two battlegrounds around Barnard's Loop). The more distant Guardians sites post-date their conflict with the Thargoids, from the Guardians lore, and are in regions where no Thargoids or Barnacles have been reported. So I can understand the Thargoids considering us an existential threat and sending fleets into the Bubble, whilst trying to avoid antagonising us or drawing our attention to areas behind the lines.
The Guardians' conflict with the Thargoids was a defensive one (the Thargoids may have perceived their territory as being threatened but the Guardians had no choice about where they arose). They were protecting themselves. Humanity's conflicts with the Thargoids have both been offensive - it is the Thargoids who have been protecting themselves.
So I don't understand why the Thargoids would have a stronger response to the use of Guardians technology than to our own (although even the non-Guardians AX weapons supposedly use Guardians materials). Unless: the Thargoids think that the Guardians AI constructs are still around and are mistaking our actions and technology for a Guardians AI deciding that they need to be wiped out as well. Or maybe they perceive us as agents (whether aware of it or not) of a Guardians AI. Perhaps they have had hostile encounters with Guardians AI in the million+ years since they were created and we have not found the archaeological evidence yet.
I feel that we need to better understand the relationship between the Thargoids and the Guardians, else destroying or disabling a Titan is likely to escalate the conflict further. It might even antagonise the Guardians AI, if they really are still around...
Regardless, my guess is just that they still destroy Guardian tech because they know it can be weaponised against them. Human tech can as well, but I guess that's one step too far, like the Guardian tech contains some special kryptonite stuff whereas destroying human tech would be like destroying anything containing metal because we could make a knife out of it? I dunno, thats just the vibes I get. Or they think the Guardian tech is dangerous to EVERYONE, regardless of our relationship with the goids...the Guardians did wipe themselves out, after all.