Bungie has made some statements that there is a single player in this game. But is it truly a single player game or is there always other players running around?
It is single-player in the sense that all of the story content can be completed without PS+ and/or without interacting with anyone else. There are also instanced story zones which will never have random people running around.
People are expecting high level graphics from old and outdated hardware, we have seen how far that the previous gen can be stretched. A lot of these screenshot comparisons are relying on two assumptions; first is that these are actually the same locations in the respective screenshots, looking similar does not mean that they are the same location and the possibility that a few areas have similar architectures is highly probable, the second is that the original demo visuals were run on a last gen console rather than a PC (there is no concrete evidence either way). Finally, a lot of people's anecdotal opinions are based off of twitch streams (horrible quality even at highest settings due to compression) and the small amount of game-play video already uploaded, both of which suffer from compression and only illustrate a small portion of the final game.