Thanks, Eva.

I believe it's for example when you're sitting in a room full of people, but nobody is paying attention to anyone else in the room, all are transfixed by their smartphones, playing games, chatting, etc. but not with each other, and not necessarily with a person either (it could be a chatbot, a machine, a game). They are alone, but physically together.

I get it. Thanks, Holly :D

It is probably better to think of the "together" part. You are with another person. But only that other person. So the two of your are (alone) together. This is in contrast to being within someone with other people around.