Actual star systems to look in are a bit difficult to get, since the previous games used a different setup for space, after all, the first Elite had us going to multiple different galaxies, not just moving around the Milky Way itself. A ship was found around Polaris, from whence you get transported instantly to the Miacke system, over 1000 LY distance, as Thargoid hyperspace technology is WAY more advanced than Humanity's, and they may even come from the dimension that we call Witchspace, we don't really know. We do know the Line was held over towards Polaris, not towards the Pleiades, as that was the direction that most of the Thargoid interactions with Humanity were taking place. Which doesn't mean a lot since we're not even sure they live in our own dimension to begin with, although INRA was pretty sure they had a homeworld in the Milky Way, so...
They are evidently ammonia based insectoid lifeforms with a hive mind, all female, and they aren't all the same, it would seem there are different...branches, sects, not really sure what to call them, of Thargoids. We ended the last game being at peace with them and having technological and cultural exchanges.
David has stated that any peace with the Thargoids may be short lived, so they may well show up as enemy forces. We do know their technology however, so the UA's should be from some OTHER alien race out there, which we do know exist, Humanity has wiped a few of them out as I said, and David was clear, there are other alien races out there and they'll show up in Elite Dangerous.
I highly recommend this site for more information, Drew is a good source of information and has published an official Elite Dangerous novel, so he's got some good intel. http://www.drewwagar.com/progress-report/thargoids-the-definitive-guide/
While I do agree with you, I feel the point that "we know their technology" is not a strong enough one to make it a certain thing. it's been 50 years since we last had contact with the Thargoids hasn't it? technological breakthroughs can happen in much less. Thinking of humans here on Earth, and an example I'm familiar with; audio amplifiers used to all be valve amplifiers, it took much less than 50 years for us to move over to the digital ones we still use to this day. If you were an alien race and looked at the valve amplifiers, then at the digital ones, you would struggle to see all too much association between the two. Add into the mix the fact that, as you say, they had different sects, and that they surely did not share ALL their technology with us, and it becomes much less certain that we can be sure they are not thargoid.
That said I am in agreement with you that they probably aren't, I just am not completely convinced.