It predicts random, unpredictable calamity. It doesn't specify who that calamity is for and both of our other visions involved forces which wouldn't necessarily effect us directly until after they concluded. I'd wager any calamity related to these two is aimed more at the imperium of man than us.
one persons calamity is another's golden ending
Supposedly there are actual webway routes that can enable time travel. Just as far as I'm aware they are the kind of thing that GW says, it's a plot device that's only going to show up for this series so we are going to allow our (typically Imperium) protags to use and say that for whatever reason the Eldar don't and in fact can't.
I'm so appreciative that I'm able to learn this all second hand rather than experiencing the asswipery that seems to be typical GW plot contrivances. I'm definitely not in their consumer demographic, and I like it that way.
If it involves giving Choas any kind major advantage for the millenia to come, that's bad for us regardless.
Excusing a small list of exceptions, I don't think anything Ahriman does is anyone's idea of a golden ending.
I doubt a pair of echos of a primarch and a particularly noteworthy legionary are going to do anything which leads to a multiple millennia long chaos advantage. It's definitely not a certain thing between them not even necessarily being pro chaos versions of themselves before we account for a much changed galaxy they'll have greater difficulty navigating without clashing with whatever Aeldari federation of craftworlds is active in the region.
Hence why I've been using words and phrases like "probably" and "if it involves".
These were reward interludes for the quest getting quest of the month. I'm pretty sure their actual premonitions in quest that inform us of potentially events that are still in flux which we might influence, but also aren't required to deal with if we don't want to.