Playing Devil's advocate here (or a thesis review panel lol):
It may not have been the Guardian AI that evolved into the Thargoids, but there still must be some significant link in the past between Thargoids and the Guardians that goes so far as to make the Thargoids react very violently to their technology in a way they just never have to us.
Oh, I think there's link(s) but the Thargoid's reaction to the tech might be technological/medical (since they are hybrid forms). Imagine something similar to penicillin, which tries to block the division of bacteria - this might be a very bad thing to a biological entity that seems to have regenerative powers. Even if not encountered before, there would probably be a very harsh reaction, just as, instinctively, humans would spit out turpentine without knowing what it was, except "bad."
Your second point about the Guardian link with Humanity:
There's so many factors that guide biology over an evolutionary process.
Also, we've discovered organisms that use silicon in place of carbon and can convert chemical into energy by chemosynthesis
on our own planet (hydrothermal vents). So there's no reason life must evolve in a carbon-based form.
It's simply that we can understand each other. Ram Tah fairly easily translated the Guardian data without any frame of reference.
They use morse code
I am still questioning this line of thinking. Despite us being very similar to ancient Egyptians in body, mind, social structures etc., until the early 1800s with the discovery & translation of the Rosetta Stone we could not translate Egyptian hieroglyphs. It seems too pat that this data was translated so quickly, and only by one man, and not a committee like I would suspect, with the Feds, the Empire and the Alliance all demanding the original messages to decode themselves.
And I STILL do not think Morse code (Federation Blink Code), which sticks out like a sore thumb, is from Thargoids.
Same with the Thargoid stuff, it's all things we can understand. <snip>
I think this is a red herring. DB keeps saying, "the Thargoids are truly
alien." I think it's much more than having four hearts...