If I were to guess, the grass seeding is relatively inexpensive. So even if it's very temporary, they'll probably get their money's worth off of having beautiful aerial photos posted on the news and social media (compared to having a bunch of dirt).
They may also not be fully committed to what project they move forward first. And so instead of leaving all the expansion pads as dirt, they seed them all and figure it out later. 'Just get the park open and pretty" is probably what they're thinking about today. That doesn't necessarily mean expansion isn't coming soon after