I didn't see it in the article, and so I wonder what the country breakdown for this might be. I get the impression that in the developing world, where android has really taken off, the ability to receive updates is diminished.
Importantly, in China, the biggest Android market, none of the phones access the Play store, so that entire country will be omitted from the data. I'll bet they're not getting updates either.
In Germany most cheap phones (about 100 € price tag) that you can get in big chains like Mediamarkt, are still running 5.x variants.