Or is this really just charging money for all their now open source contributed content...
They will be selling additional "premium features" at launch time, as described in this article, and plan to sell specifically created additional content, like in-depth articles, in the longer run, as described in previous articles.
Maybe this will enable them to hire back some of the people they let go (assuming these people are willing). But the main goal is to put mdn on a level of funding where little or no funding from mozilla is required anymore.
Cynical take: because reading is hard so people just don't do it, and see "MDN" in the title so take this as their chance to scream into the void about whatever tangentially related nonsense they care about today.