[SPOILERS] No one seems to be talking about that OTHER huge event

[Iscah]

But all of this happened on it's own without any interference from us or the Scions. If Zenos stopped Black Lotus, then how did the dark timeline that lead to Gra'ha Tia becoming the Exarch materialize?

How can we be sure he stopped it from ever being used? In a hypothetical timeline where the First is still primed for calamity and Zenos is in charge, the situation that led to the use of Black Rose could still happen, and Zenos might not have the final say as to what his forces out in the field do in a moment of panic.

In the original timeline the WoL wasn't janked out of combat against Elidibus and might have just destroyed the Zenos body right there, eliminating Zenos's motivation to travel to the capital and thus his killing of Varis simply not happening.

It's all very theoretical and wibbly-wobbly, especially at the moment, but we can't say where the timeline splits until we are definitely past the point where the Calamity would have been triggered, but now has not.

The "original timeline" (ending in the bad future) may have always incorporated the Exarch and his attempts to contact us, while the split point could depend on whether he succeeds or not, whether we successfully complete our quest or not, whether we win the final battle or not. It might even hinge on some future point that we won't see until the patch quests.