Yes, or someone did show up despite knowing the risks because they trusted Hawking to understand the dangers of revealing the secret of time travel and not sharing it with any living soul. If time travel were to ever become possible and somewhat commonplace then the chances are probably close to zero that everybody chooses not to attend this party (assuming the invitation remains famous for long enough). Perhaps the party was crowded with people thinking the same way.

It's much more likely that it all just played out exactly the way Hawking said it did, of course. But it's a fun thought experiment to play around with.