I'm pretty sure you're correct. The core is the "skinny waist", Rounded corners = floor plate. I can't imagine why a core would have rounded corners quite like that.
These buildings have a unique structural design due to the buildings overlapping the train yard below. They installed huge concrete "bridges" over the tracks to avoid pillars going down between the tracks. They bridges are very strong, but they can't support a skyscraper, or even part of one. So they placed the building's cores over solid ground, and put all of the weight onto the cores, so none rests on the bridges over the tracks. That's the steel structure they've just completed and shown in the photo.
This shows the core design:
...from this video: