I'm not sure the photographer did it here, but if I want to do a shot with no people I do it like this:
1. Use a tripod
2. Take many, many shots of the exact same scene.
3. In Photoshop, make a layer out of every shot.
4. Erase only the people in every shot, leaving an empty space where you can see through to the layers below.
5. If you have enough layers, you will eventually see nothing but the scene after the people are erased.
(This only works when people are moving through the scenery. If people are standing and sitting for a long time it will not really be possible.)