If you notice the comments on the video link you posted, others had the same missing slot. I believe it is only on the MB if you ordered a system with 2 M.2 slots initially. There were obviously 2 variations of the MB. And no you cannot modify the board yourself.
Hi  Why does my laptop not have a ssd2 socket? Watching this video, I thought it will be possible to add a 2nd ssd: [URL] Here a picture from the inside: [IMAGE] Is there anything I can do or do I need to accept that I cant add another nvm.            
Dell Precision 3560, SSD2 socket missing