The video seems to be real, because the location EXACTLY fits the know murder spot. It does not seem likely that such a video could have been prepared in advance.
The murder EXACTLY takes place when the cleaning vehicle passes by. It is absolutely unclear where the assassin come from. Nobody can be seen walking towards or behind Nemtsov and his companion. Neither can somebody be seen walking to that specific spot to wait for them.
Therefore it seems very likely that the cleaning vehicle has something to do with it and the assassin could have traveled with the vehicle.
Since Nemtsov was shot from behind and the cleaning vehicle already caught up with them when the murder took place, it does not seem likely that the assassin traveled on the passenger seat next to the driver. He had to open and close the door and that may have drawn their attention. It seems likely that he had traveled on the back of the car. He could easily jump on and off and could also estimate if there were other people to close to them to do the operation at all at that moment.
Using a cleaning vehicle gives a lot of advantage: you can drive slowly, draw no attention, wait if necessary and thus pick exactly the right moment (with no other people nearby).
It does seem like the assassin had been following the couple already when he jumped on the vehicle. They may have crossed the road where the cleaning car was waiting. After seeing them pass by, it started entering the bridge. When they passed the assassin, he jumped on. In this way they could be sure that they had the right person AND they could pick the right moment without running to catch them up.