Definitely interesting… not too surprising that there has been a bump in mobile reading over that past few years - seeing as everyone's phone screens are twice as big as they were in 2012. Anecdotally, I am more likely to read on my phone now than I was a few years ago (I always used to reach for my iPad before I had an iPhone 6).
When reviewing these stats, we should keep in mind the primary use case of Wikipedia - a reference. While it is true that some will read significant portions of a book or a blog posts on their phones, most people aren't looking to read a Wikipedia article from top-to-bottom. Some will read a section or 2, while many others will only need to ready the first paragraph to get the answer that they need.
So even as the number of "long form readers" increases on mobile, that might not directly translate into more "full article Wikipedia readers" on mobile.
I definitely believe we should continue improving our mobile reading experience - it will only become more important as these numbers increase, however we shouldn't draw to many conclusions from this article as the content being discussed is quite different.