But they don't indicate that. The most you can infer is that first contact maybe happened around then. It does not indicate who shot first, that's just your assumption that it must have been the Thargoids - but that contradicts their later behaviour at Veliaze, and what we've seen in the Pleiades. It's always in response to something else.Of course, the part you quoted sounds fairly reasonable if the Thargoids have previously spend their free time mucking around in witchspace waiting to ambush unsuspecting pilots.
Regardless of what is playable and what isn't, earlier reports of mid-witchspace malfunctions would further indicate the Thargoids fired first (or, if you want to be charitable, scanned them and left the innocents on-board to die, which is probably considered a war crime).
If they were already willing to randomly start attacking in 2800, why do you think they stopped doing that by Veliaze, or the initial hyperdictions in 3303? They could easily have just kept doing that and never given us warning of their existence. This was already a plot point in FFE.
The shutdown pulses of their ships specifically spare life support. The entire process of shutting down a ship and scanning its cargo would be completely unnecessary if they were actually willing to attack for the hell of it. The only action doing scans enables is to spare uninvolved ships. But then, hundreds of years and one war earlier, they'd just shoot on sight? But they stopped that again for Veliaze?The Thargoids had the technological capability to destroy human ships with ease and it did not evolve over-night although their appearance in our Universe was sudden and dramatic. We can rationally assume that they could have destroyed all the early probes and less well-protected ships at an earlier date had they chosen to do so. They did not.
Anyway, I still don't agree with the idea of damning a species based on who shot first even though I think it was humans. There are so many more factors than that, also I don't want to have to hand myself in to my local Titan just because some random pilot panicked in the 2800s and started blasting, or because of Veliaze.