"if Google wanted to, they could parse the external links from a Twitter stream without actually following them" The problem is that there are no external link

"if Google wanted to, they could parse the external links from a Twitter stream without actually following them" The problem is that there are no external links in the tweets. All tweets point to t.co, which is run by Twitter. You can't find out the ultimate destination of the link without following it through Twitter's redirector.