Not crazy, just under a misconception. The Vex can travel to the alternate timelines but they don’t travel back in time in our timeline. If they could, they’d have won already just by going back before anything evolved and converting the cosmos as it forms. Why let your enemies evolve and harness paracausal power when you can stop them from ever existing? Here’s a breakdown on time in Destiny, based on what’s been said and what we see/don’t see happen if you’re interested. The best info on how time works (lorebook Unveiling) implies it’s the Many-Worlds concept: one primary timeline and everything that doesn’t happen but could branches off, and from those branches the process continues. An infinite branching with only one true timeline. Season of the Dawn and Elsie’s Dark Future lore shows that actual time travel requires practically god tier power and control. In Dawn we scrounged up enough power to alter the fate of one person in an area technically not in space/time but without the finesse we literally broke time and almost destroyed the cosmos. So a minor change in a space that has flexible rules and we still almost ruined everything. In Dark Future, we see the Traveler/Gardener rolling back the entire cosmos and everyone in except Elsie, so much more power and control involved, so many times Elsie lost count without a hiccup. The difference between what mortals can do and what the god of creation can do is ridiculous. And it’s a smart move. As cool as time travel would be, it wouldn’t work narratively with our enemies. If they used it once, we’d lose. In fact the expansion Curse said something similar. That if the Vex could time travel as we think of it, how haven’t they won already? I thought that line was put in to be a tip-off for us.