I've worked in the publishing industry in the UK - it is experiencing long term decline. To illustrate this, see the multitude of publishers merging, cutting staff and closing imprints. Very few publishers are making money any more. The story is the same for bookstores - there is one significant UK-wide chain now (Waterstones).

The article you cite doesn't even show healthy figures - it shows a bump in 2015, then fall to almost zero growth in 2016 for the UK, and a dramatic decline in growth for the US from 2015 to 2016. Hardly signs of a healthy industry. This is just the kind of cherry-picked data the industry has been deluding itself with for years. The figures produced by the industry are unreliable, and the interpretation of those is equally suspect, as evidenced by the article you've found. They've tried to manipulate stats by doing things like pricing ebooks higher or just below physical books for example.

Barnes and Noble will close soon (as this article predicts), as will all the other big brick and mortar booksellers, and soon after that mass-market paper books are doomed as Amazon has little incentive to promote them over ebooks, and no interest in sustaining publishers.