"Another challenge was building an AI that knows the difference between a fact and an opinion so that we do not flag opinions as false, since only facts can be false."
In news media that is typically considered credible, opinions are very often presented as fact, often marked with "an unnamed official said" or "our anonymous source told us."
Much of the news regarding the outside world is gathered at the State Department Press Briefings, where the Press Secretary will him/herself admit that what's being communicated at the stand is the opinion of the US government, after it goes through an interagency research, vetting, and message coordination phases.
Very often these coordinated opinions are then presented as fact at CNN, the New York Times, the Wall Street Journal and others.
Of course, this relationship led Americans to think that Saddam Housein had something to do with 9/11 and justified the invasion of Iraq, led the media to report on Mass Surveillance with the opinion label "Bulk Collection", led to inaccurate coverage of Bahrain, Kosovo, Benghazi, Jessica Lynch, etc. President Obama's National Security Advisor publicly bragged about the ability of coordination within the interagency to create an echo chamber in the US press.
I don't think there is an algorithm that can tell the difference between fact an opinion, unless somehow that algorithm is an automated investigative reporter.
It seems insurmountable at this stage of technology to try to separate US government propaganda or propaganda from interest groups from facts.