Later they demoed they can track your interests in what you've read on the web to show you personalized news on their news app and keyboard autocompletion.
And if this is true, this is a huge plus, because nobody else is doing it. Because Windows is cloud-based for this stuff, just like Google. None of the 'smart' features of Windows 10 work unless you enable what amounts to a keylogger and the ability to send a lot of extra data about what you do to their cloud service. (Well above and beyond the telemetry most people worry about here.)
That'd be first-party tracking. If I'm using an Apple app I expect Apple to know it, but I don't necessarily expect Mixpanel or Google Analytics or Segment.