Mixing in-game and out-of-game observations - if Thargoids feature in this game, Frontier will likely update them and their technology, visually. What we expect of aliens and their ships has evolved greatly over the last 30 years, just as our own technology has advanced. If they do this, they likely will need a lore explanation for this change.
Cue the article in Galnet talking about how branches of a civilisation from different times and places could seem entirely different and yet be related. This, to me, is Frontier saying "if the aliens in the game aren't what you expect, it's because they're an offshoot civilisation, but they're still related".
This also means that the scientific 'rejection' of Thargoid origin of the UAs might not be the conclusive slam-dunk it appears to be. It might just mean the artefacts don't correspond to the technology/materials of the Thargoids we've met; but may well correspond to the Thargoid civilisation we're about to meet!
From MB:
Perhaps this "divergent" branch of Thargoid civilisation is actually the "mother species", the main branch? I know pretty much nothing about Thargoid FTL capabilities, so I don't know if there are time dilation effects; which might impact on the technological differences/evolution between branches of their civilisation.
All supposition, of course.
I'd look at the divergent species very much the same way I'd look at the evolution of the common honey bee vs. the Red Wasp or the Hornet.... One is benign if undisturbed, the other aggressive when attacked the other plain simply evil...