The only way I can read this is as an admission Google+ didn’t take off in the way it wanted it to, despite being mandatory to use other popular Google products, namely YouTube. Google now focusing Plus on the small niche discussion communities where it is doing well (which are good) and leaving things like YouTube as independent, separate, products.
Although the company has been discretely signalling this transition for a while, like Google Photos being positioned as a standalone offering, this blog post is confirmation that the dream of Plus as the persistent glue that connects your Google life together is indeed over.