I agree that the math just doesn't add up; there isn't 2 million SF of downtown office space available. I think they have something up their sleeve, and hopefully it's good news.
They just signed an 18-year 400,000 SF lease at 50 Fremont, and have the option to
buy the building in 5 years. Even if they eventually take over that whole building, that's only 900,000 SF or so. But, right across the street from 50 Fremont is the site of the fully-entitled
350 Mission, which would offer an additional 350,000 SF. Nearby are proposed towers at
50 First and the
Transbay Tower, each of which would provide over 1 million SF. Of course, none of those are built yet, which doesn't square with their claim that they "need space faster than it can build it."
For Mission Bay, this is a definite blow in the short term. Long term, I think it may be a good thing. The Salesforce suburban office park never felt like a good fit to me.