Well if you and I are thinking that the people running Twitter are dumb and left a billion dollars on the table, there may be more to the story.

I don't think they're dumb. The stock market is perhaps the most analyzed and quantified human endeavor in history, and at the intersection of science and business I don't see any reason why the explanation isn't likely to be exactly this simple.

Besides, it's not leaving money "on the table," it's handing it to non-company insiders, for the benefit of those who sold into the doubling.