That kit should make up for some of the D850's weaknesses, namely the lack of focus peaking in 4K and poor contrast-detect autofocus for video. The Ninja Flame recorder supports focus peaking, and Nikon's FX lenses offer excellent manual focusing with an automatic AF override when you grab the focus ring.
The kit should thus tempt videographers that might have been considering the Canon 5D Mark IV. The latter significantly crops 4K video, while the D850 doesn't, using intelligent line skipping to minimize moire. On the other hand, the Canon 5D Mark IV has excellent video autofocus, and despite the D850's excellent phase-detect photography AF, its contrast-detect video AF system is pretty bad.
In comparison to Sony's A7R III, however, the D850 doesn't hold up as well, as the A7R III has both a no-crop 4K option and decent autofocus system. Still, the D850 is currently the world's best DSLR, and for photographers that also shoot a lot of video, the Filmmaker's Kit is saves you around $700 over buying the parts separately.