Time travel will be highly controlled by the authorities, probably that party was surrounded by future cops looking out for rogue amateurs.

More like that you need tons of resources, skills and knowledge to do it. Nothing a private person could do.

But if the time machine itself travels through time, an unbounded number of people could use the same machine at the same time

I guess same way no one has used a nuke or even a dirty bomb yet, even though the latter would be technically feasible for even a small group.

But how do we know no one showed up? Someone could have showed up, stuck him in a time machine and took him to the real party then zapped him back to his time not a second later. :conspiracy: