Regardless of embellishments, it clearly has elements of truth that are immediately relevant considering the interaction earlier. Plus most creation stories aren't told by an individual who is what, a few generations removed from it actually occurring? Even if you don't rank it as the complete and unaltered truth, its certainly something to make note of, no?
That would make sense to me, forgetting the exact wordings and such could probably occur over that length of time, still I would assume that something would transfer over.
Going back to the original creation tale, spoilered for readability,
In here deathfang refers to the dragons and old ones visiting the world, which was then an iceball, through the "void". That what we now know to be the old ones traveled through the void in the "silver ships". Its stated that the void is outside of the planet, and the dragons could visit and travel to other worlds by going through it, as well as that they could go through it to visit the moon.
In this last (half?) update, what Tochter says is that
Im struggling a bit here with modern assumptions of space travel and whatnot, but I think it would be reasonable to connect "come from beyond" "silver ships" and "nurture Her land" with the story of them arriving from beyond this world through the void in silver ships to make the world habitable and not an iceball. Maybe not entirely and certainly not definitively, but it suggests something that's a different story than traveling from Ulthuan in the ships.
Its not impossible to consider that they would use the same ships later to cross from Ulthuan later on, but I think that it does raise some red flags, and knowing information which could also fit the translations makes the proposed explanation regarding Ulthuan rather messy. In the end there are just extremely few data points about the old ones and their silver ships to work with, and when what is essentially our only major amount of information about them provides information which seems to correlate with Teclis' translations, I guess it just feels a bit odd to hear a theory which doesn't take any of that information into account, not that Tochter would have any way of knowing