Let's start with who FD would like to implement this for... Is it going to be: A) only people who have read the official books, and know the lore about the game. B) only people who have read the 1.2 patch notes. C) everyone in game, regardless of their knowledge of the lore, or the patch notes.
I hope you said 'C', as that is what would make most sense from a design perspective. So we can automatically, and logically discount anything from 'A' and 'B', that don't have 'C' in common. With me? Good.
Ok, now we're getting somewhere. So if they want this to be accessible for everyone in the game, and we assume they want it to be found, how can they let the player base know about it, without handing it to them on a plate? A) Hope the player base run into it randomly, in a playable area that is at least 400 billion separate instances. B) Give only Vague comments on the forum, and hope the entire player base reads it. C) Give them a clue in game, that everyone has a chance to see.
I agree it would be designed for everyone. I don't agree that implies that everything out of game should be discounted. Being designed for everyone means that anyone can find it, and all signs point to that being the case since anyone could stumble across the right WSS or SSS. That does not mean that FD will not provide additional hints out of game. The 1.2 trailer is an obvious example that could well be a clue. I also don't think it makes sense to discount statements made by DB and Michael just because they aren't in-game. Unless you think they're trying to mislead us?
My personal opinion is that the UA is not the start of some chain of clues that leads to Thargoids (or something else equally momentous) in game right now. I think it's just a taste of things to come, or at most a clue that will only become relevant later.
Now we're really getting somewhere. So what clues have been released in GalNet post 1.2 implementation, that involve things that could be to do with an unknown artefact? I know of only one:
https://forums.frontier.co.uk/attachment.php?attachmentid=30574
Note that article talks about "illegal technology", "alien weaponry", "biomechanical enhancements", and "hostile AI". It's not unambiguously about UAs. It's also only two weeks old. Wings had been out over a month at that point, with the trailer revealing the existence of UAs. That isn't consistent with your position that FD would only hint via in-game means and not expect people to just randomly stumble across them. If your contention is correct FD would have provided a hint almost immediately after 1.2 was released, no? Interestingly, the "weird science" story is not in the Galnet archive (
https://www.elitedangerous.com/en/galnet/17-APR-3301). Whether that's intentional or accidental, I don't know, but it does mean that it is not available to all players any more. It's only available from screenshots posted in the forum and unofficial 3rd party Galnet archives.
Why do you say any clue must be post 1.2? Don't you think FD might have been planning this for longer? I don't think you should discount things like the galaxy map description for Peregrina (which mentions alien artefacts) so quickly.
Another example is the Galnet story about Soontill relics, which predates 1.2 by month. It's probably not directly related to UAs but it is the only Galnet story this year that has mentioned Thargoids (and incidentally the band "Jjagged Bbanner" which is a nod to the first mission from FFE). Again I think it's foreshadowing for a Thargoid storyline that FD will introduce later.
I do agree with your conclusion. Everything plausible points to UAs being found in signal sources inside inhabited space.