For whatever reason, this was only announced on dev.to and not Stack Overflow.
In a nutshell: yes, this is an official partnership. The idea is to make DEV, a social blogging platform for developers, the place to have extended discussions about a Q&A's subject matter so as not to clutter up the main Q&A. Or just write anything about the Q&A, really, since sharing to DEV essentially just creates an embed with an excerpt of the Q&A in the post composer.