To understand the context here readers need a bit of history. RedHat is, of course, a company that has been wildly successful commercializing open source software (in particular Linux). It is also a big part of the OpenStack open source cloud computing initiative. Mirantis is a company focused solely on helping companies move to OpenStack. These two parties were once upon a time best of friends with Red Hat making a significant investment early in Mirantis’ life.
And then things soured, to the point where Red Hat started telling customers that it didn’t support its own Linux distribution, RHEL, on Mirantis’ flavor of OpenStack. There was lots of back and forth, and lots of minutiae around the move, but essentially it indicated, very publicly, that Red Hat and Mirantis’ bromance was finished forever.