We can assume that, if it is Zodiark, he is using Arbert's voice because it is familiar and therefore likelier to be trusted. Arbert gave us - and even Minfillia - a lengthy speech on the dangers that beset his world. This may serve to establish a reminder of those warnings.
Even so, the voice is distorted, suggesting it requires quite some exertion from him and there is a great deal of urgency in it - this is my favoured interpretation of "eon become instant", because I don't think we're going to see time travel or timeline/world mergers just yet. There's strong pointers that Y'shtola may have been a bit reckless with that Allagan device, especially with the optional NPC dialogue after the event. However, it's a singular event and probably just a trigger. Going back to 3.4, one must reflect on Arbert's words that serving the Light, he and his fellow heroes did "everything right" and still, their world shifted from being endangered by Darkness to a flood of Light. Whatever the Ascians' motives, I don't think we fully understand the nature of either their god or Hydaelyn, or the Light and Dark, as such. I am reminded of Arbert's comment that it's not so much about Light or Dark as it is what you do with it.
I agree with you that someone - possibly Zodiark - is probably trying to warn the Scions that their plan is rather bad. It's transparent, it is a risk Elidibus will have calculated the minute he took over Zenos's body and won't harm the Ascians, who thrive off chaos, so much as it will destabilise the Empire and along with it, many innocents' livelihoods.
As a general comment on the aspect of time compression, and the whispers in various venues about how this might entail time travel, I rather doubt it. My view is that it's possible that if this is Zodiark, he may be able to view time as a singular point/moment, and therefore speaks in a way that might seem like travel, to regular mortals.
I am linking the video because I think the themes in it are about to be realised in upcoming patches/5.0.
https://youtu.be/SMMr_BZddAw