What I think is this - when they said "Internet", 3-4% users understood that to be open Web, with browser, a .com and the whole package. They responded correctly that they don't use it. But correctly identified that they spend time on Facebook. So in users' mind, they correctly distinguished open Web from the apps. However, they conflated the term internet with the web.
And a lot of people will make such distinction. If you don't do any Web browsing but remained in contact with your family over Whatsapp, you will term it as, "I was off the Web to save time, but I did use Whatsapp." That does not mean what this article concludes, that you don't think that Whatsapp uses Internet.