Keep in mind that construction started in earnest in January, and they've built substantially all of the operations area, plus half of the west stands in that time. Given another 4 months, it's not unreasonable to expect them to be able to finish the other half of the west stands. Also, all of the seating bowls are precast concrete, which can be placed much more quickly than concrete poured in situ.
Also, I'm inclined to think that the timeline everyone references is not entirely representative of the actual construction schedule. For one, it shows the upper deck being completed before starting on the lower tiers of seats, even though the upper deck won't be open this fall.