Is this maybe a "sell high" scenario for twitter? Sounds like the article is suggesting that, while demand is still strong, the supply of available office space is rising.
Perhaps their thinking is to get the asset under lease while the price is still at it's current rate and then spend on expansion later if it's necessary (at theoretically lower prices based on the supply trend?).
The current tech boom started around 2011-2012, and a lot of companies signed multi-year leases around that time. My company leased an office for 5 years in 2012, and our lease will expire in early 2017. We expect the price to increase substantially, and we are considering moving the office to save some money.
I suspect a lot of 5-year leases are going to be ending this year, and a lot of companies will be looking for new space. If Twitter might as well get a piece of the action, if they aren't using that part of their building.
Perhaps their thinking is to get the asset under lease while the price is still at it's current rate and then spend on expansion later if it's necessary (at theoretically lower prices based on the supply trend?).