Their encounter with the Guardians was similar to ours, whereby they had seeded worlds with barnacles for meta-alloy production. The act known as "Seeding" is in Thargoid principles a territorial marking/claim. When they returned years later and found that the Guardians had now colonized those worlds they seeded, they immediately opened fire without mercy. The Guardians at first tried a very calm response and suffered a lot, until diplomacy with the Thargoids was deemed impossible (As the Thargoids refused to speak with the Guardians even tho the Guardians had deciphered the Thargoid language) and subsequently deployed automated weapon systems that proved devastating against the Thargoids, ultimately forcing the Thargoids to retreat from their conflict with the Guardians.Yes I would agree with most of what you say.
The simple fact is humanity has gone from a relative state of co-exsistance to one of aggression from the Thargoids.
The Thargoids would be warey of other species after the encounter with the Guardians, that with the hostility coming from humans reaffirms their suspicions especially after recent events.
But this is just my PoV.
Humanity never really had a co-existence with the Thargoids. Our encounter with them that sparked the first war was the same as what sparked their conflict with the Guardians. We deployed the Mycoid virus, which repelled them, allowing Humanity to push further, believing the Goids gone. In 3303, the Goids returned and saw that humanity occupied the Pleiades. Queue round 2 boogaloo ever since.
TLDR - From viewing their original encounter with a peaceful non-warlike civilization like the Guardians it can be deduced that the Thargoids are a real threat and that humanity, whilst making the error of mindlessly colonizing seeded worlds without analyzing the consequences (Looking at you, Sirius Corporation, Coalsack colonies), should not withhold from repelling the goid threat.