The way I understand it is that there are both legal and illegal phones. The legal one is what the article is talking about. They contain all sorts of crap (spyware and the lot).
Illegal phones can ring to the outside world, be used to watch illegal media and so on.
The legal phones are just Chinese phones rebranded and have severe lock down. Here are some of them https://www.businessinsider.com.au/north-korea-smartphones-2018-7?r=US&IR=T
North Korea has it's own carrier. https://en.wikipedia.org/wiki/Koryolink
I think the phones are sold at official department stores. They're marked up significantly in prices. You're right in saying that most people only make around $1-2k. The people who are buying (officially) are probably those with black market funds and the elite.
Here's a video from an Indonesian dude in North Korea. This is their tech store: https://www.youtube.com/watch?v=FtDWYzwwuUs
Are cell phones heavily subsidized in N. Korea or are they provided by the government? I remember seeing once that the average worker there only makes $1000-2000 a year so I don't see how they could afford to buy a smart phone in the first place.