.... If the lenses are too light, you wonder how robust they are...
Looking at the differences in optical design of the 400/2.8's, I'd say the vast majority of the weight saving would be simply the reduced glass sizes, with the assumption that their diagrams are dimensionally equal.
It would be unlikely that they compromised structural integrity in any way.. more likely improved it.
If the suppositions and rumours of a higher end pro model Nikon Z cameras are to surface prior to the Tokyo Olympics in short term future, it's very likely that Nikon will respond to these new lenses at the same time.
Nikon do have 3 unmarked Z lenses slated for a 2020 release in their roadmap. The question would be if they would they do long tele Z mounts lenses, or maintain the status quo and keep them in F mount(if they did respond)?
In saying that too it's probably very likely that Canon will also have a high end pro level RF camera too.