Chevron has a huge footprint downtown. They bought up the old YMCA specifically for this build, but it was put off for a few years due to the economic downturn. Their data centers are also capped out in the old two Enron buildings because they can't get more water to cool them, so they need this new building badly (I'm surprised they are waiting longer). They also have about 50% of the Continental building, or did back when I worked at Chevron in 2008.
I wish we would build a sky scraper 1836 feet tall just to stick it to NYC and claim the tallest building outside of the Burj Kalifa. (off memory) 1836 is the year Texas won its independence, as One WTC is 1776 ft
they still have that space and iirc they also grabbed 300k SF of the Devon sublease space in allen center. wouldn't be surpised if they added a little more to make sure they have enough space until the build materializes.