Yes, Stephen Hawkins is a serious man with a lot of scientific knowledge. He wouldn't make jokes on such a serious matter. If he says that our humanity has about 1 000 years left, then we believe him. If he says that the party took place, then we believe him. The reason no one showed up on this party is that no one really wants to know the real truth about the time travelling.
Stephen Hawkins has a brilliant mind, but he has also had some far out ideas lately. He has said before that he belives time travel and other radical ideas are possible and real. He also is pretty convinced that we can prove a negative. No matter what I don't think it was a joke.
Yes, Stephen Hawking was serious about his time travellers party. Stepehen Hawking is a genius, so he's serious about anything he does. This party was supposedly held on June 28, 2009 was actually an experiment. He wanted to see if anyone would actually travel back in time and show up at the party. No one did, hence proving his theory that time travel is not possible.
I think he meant it as kind of a tongue in cheek joke, especially since Hawking claims to have empirical evidence that time travel isn't actually possible. It's makes sense viewed in the framework that he called it a time-traveller party experiment and sent the invitations years after the party was actually held. Unless of course it actually did work but in a parallel universe.