In addition to benefitting from this open source work, is SE giving back to it and helping the FOSS ecosystem? In particular:
Did SE just fork the editor, or are they contributing back to the main project the new features they add for their internal use? From the contributor graphs of the two projects, it seems that the answer is no, at least for now. And I don't see any pull requests. Is this planned for the future?
Are they supporting Prosemirror's author? It is his explicit request on the project's page that business users do it:
If you are using ProseMirror to make profit, there is a social expectation that you help fund its maintenance. Start here.
(I am not affiliated in any way to Prosemirror, and I had never heard of the project before SE announced their editor.)