I was reading a Reddit post, and the author casually mentioned that the infinite Forrest reveals that he Vex can’t actually time travel, but rather can just simulate all the timelines and make a decision.

What?! Where was this said? Anyone remember? I guess that would explain why they don’t just go back in time and blow up the Earth before they traveler gets there. But it seems like a huge retcon!

It wasn’t said anywhere. That person was extrapolating and voiced a theory based on the existence of the Infinite Forest itself. I don’t find that theory particularly convincing. Being able to time travel doesn’t mean that being able to simulate infinite timelines in the Infinite Forest wouldn’t be beneficial.

We literally see ancient and future Vex on Mercury (while not in the Forest), which would indicate that they in fact do time travel.

Although everyone in-game is speculating.

Look at the Hunter Kairos Function Boots:

If the Vex had achieved what we would call 'time travel,' surely none of us would now exist." —Sister Faora, "Theories on the Vex"

Drifter also occasionally shuts it down before a Vex match.

That said, Remember Skolas, and how he was supposedly pulling Fallen through time to fight for him?
Bungie never bothered to explain that one.