The photo is actually based on a leaked press render that was uploaded by Nikon Rumors just the other day, and according to anankephi, he points out to the viewfinder section in the photo in which there are two circular cutouts on either side. You might assume that these are just screw holes to keep the viewfinder in place, but it has since been speculated that these are actually eye detection sensors that could activate the EVF in the camera.
This has also led to speculation that the D850 could be packing a hybrid system that combines an OVF with an EVF. For the most part, DSLRs usually come with an OVF, whilst mirrorless cameras sport an EVF. The argument to be made is that OVFs have pretty much no lag, but what you see in the frame might not necessarily be the same once the shutter button has been pressed.
EVFs on the other hand allow photographers to capture exactly what they see, although the downside is that if you’re trying to pan around with an EVF, you will notice some lag. A hybrid viewfinder system could offer photographers the best of both worlds, and possible allow them to toggle between either system. Like we said Nikon has yet to share any official details so take it with a grain of salt for now.