The original tweet doesn't capture the full story - we're not working on an FRCS right now because we are putting the bandwidth available for such a project into thinking about how to do an FRCS.
If you think of how we created fifth edition, we probably put more time and effort into determining what it needed to be (playtest, etc.) than into actually writing the final product.
The DMG is also still in the works - we won't even consider engaging in our next big RPG project until that is out the door, everyone has taken a vacation, and we're ready to tackle another huge project.
This does not equal:
...they don't even have an idea of how to do a campaign guide for FR yet.
The PHB just came out. The MM is coming out soon. The DM guide is still in the works but coming out in November. When a new edition of D&D comes out does the campaign setting typically come out at the same time as the PHB? I don't remember it ever being like that. Also, I would imagine that putting together a campaign setting from already existing source material takes much less time than designing a new game. I think your statement is an overreaction.