Ironically, I think at least part of it is due to less amateur content, in turn due to the dominance of mobile phones, that enable you to easily interact casually and superficially with content, but create a myriad of minor, subtle cues not to create content, just to consume it.
In a PC, in the "olden internet days" that the article misses, you had to be in front of the PC. It was an activity on itself. But creating content was easier. You had a bigger screen, a slower connection, you saved funny memes in a folder in the Desktop. When bored, in was mentally and physically easier to alt-tab into amateurish internet than to get up to do something else. So lots of people ended up losing time interactively, in front of a hundred keys asking to be pressed.
Whereas today, most of the users interact through a mobile phone with one thumb, on 5 minute intervals, in a micropause throughout the day (and possibly even during a conversation). Small screen real estate, maxed out app, no hint to alt-tab, painful to create content (you have to tap in onscreen keyboard that eats up half of your screen, you have to add the second thumb and lean over the minuscule screen, you have to go to another app to navigate a folder to upload a meme, etc). Everything takes more effort, needs to be more conscious. And when the user mostly consumes, the whole UX evolves towards that, making it easier to consume and more difficult to create, in a viscious cicle.
(I'm typing this on my mobile and it is a pain. I only see about 20 typed words, even moving the cursor back to noher word is a minor pain, etc. I just want to get it done, and I have already thought a couple of times whether it makes more sense to kill the comment mid-redaction and keep on browsing HN)
Edit: and of course people keep using laptops an pcs, but the existence of mobile phone users change the dynamics of every content community a lot.