I guess one credible explanation for how/why the Veil Jumpers came into existence is precisely that after the events of Trespasser, Lavellan and/or the other Dalish in the Inquisition took the information to the arlathvhen. Most people probably didn't believe them, but if enough people cared enough to form a group dedicated to entering old ruins specifically to look for this kind of information, then by 9:52 or whatever the year is now the Veil Jumpers specifically would have enough info to be fairly sure it's true? Maybe?