He is referring to the appearance of the Vex. (Destiny)

by Speedracer513 , Dallas, Texas, Saturday, March 14, 2015, 22:11 (3336 days ago) @ Funkmon

Hmm.... after re-reading the grimoire cards for the Vex, I think I now agree that it remains somewhat ambiguous...

The card for the Descendants (that appear on the "green" side) says: "Survivors of the Vault of Glass report sightings of Ancient Vex - ancient in the sense that they have endured for eons. Convergent analysis from multiple Ghosts suggests that these Vex exist in our future.

On the other hand, the card for the Precursors (that appear on the "brown" side) says: "Those who delve deep into the Vault of Glass have seen time itself torn asunder. Awestruck Ghosts report encounters with ancient Vex, their casings built long before the age of humanity. It would be easy to assume these Vex are the ancestors of those we face today - but with the Vex it is never so simple."

Since both cards refer to "ancient" vex that were created long ago, but possibly exist in our future, I guess we can't say definitely which is the past/future based solely on the types of Vex encountered in each.

However, I still think it makes more sense that the past was once lush and the future is barren because of what we do to them.