Something is up with that 600mm. Using the known length of 380mm for the 400mm, I get 420mm for the 600mm. Sony's 600mm is about 450mm and Canon's is 470mm. Nikon went up around 15mm from the 400 FL to the 400 Z and the 600 FL is 432mm and adapted it is around 463mm. With a TC and similar improvements compared to the 400 Z then you could see it being around 455mm maybe.
So either the silhouette is nonsense to make the image look better, the sihouette is wrong because it's still in development or it's a 420mm lens. If it's just to make them all fit on the image and they've altered the length then I think that would be very poor marketing because the shapes set an expectation. I don't recall many of the shapes changing much either as new roadmaps have been put out. So maybe it is a 420mm lens. If it is then I would imagine that it is incredibly unlikely to have a TC.
I think the TC makes the most sense on the 400 because of the sports use-case where people carry both a 400 and a 600 to the same event. If you want to win them back then show them that they can have almost the same thing but with 1 lens. I doubt many people are carrying a 600 f4 and an 800 f5.6 around at the same time. Whilst I think it would be a great selling point for Nikon if they include a TC on both the 400 and the 600, I could also see the benefit in having a 420mm 600 f4. Suddenly it starts fitting in more bags and being less of a beast for air travel. Maybe that's the direction they've taken it.
I would love to know exactly what the 600 is though before my card gets charged for the 400