Well the interesting part of this is that they will collaborate on the engine but the frontend will still be independent.

I think this is a sound strategy (if I understood it well) as the key differentiators remain but the hard parts (running the damn things) become collaborative.

This also IMO helps a lot user experience, as dealing with broken scripts, misterious wine settings and keeping up with version changes makes playing a game a big ordeal compared with other platforms (In some cases like old games, it can be worse).