I would guess DC is trying to distance themselves from the DCEU. I think Aquaman only got made because it was already in production after Justice League bombed, but I don’t think it was an accident kept the links to the other comics and word building to a bigger minimum. It clearly worked out, don’t get me wrong, but it doesn’t seem like an accident the one in pre-production and the sequel (direct or not) to the one movie that got critical acclaim and was considered the “good one” are the only two movies with the JL characters that happened. With the Batman and Superman actors leaving the whole DCEU as we know it is dead in the water. So probably don’t want to dwell on that connection too much.
I assumed that since it doesn’t seem to be a new origin movie, that the reason it is now set in the 80’s is because well… Wonder Woman is immortal. I took it to mean it is technically a sequel just won’t call back to the previous one. I mean unless they are re-doing her origin can’t say it wouldn’t qualify as a sequel.