Apparently I've simultaneously asked and not asked a question.
As to why I'm wandering through Facebook "Help" Community... well, when I created an event a couple of weeks ago I'm sure there was an option to message all guests. I now can't find it, despite Facebook's official help claiming there should be a nice obvious "Message Guests" link. And the Help Community is a complete joke because the answers are either epically out-of-date, say you can't do whatever it is you're trying to do, answer the wrong question, or are just plain wrong ("How do I message all guests?", "You click the Message Guests link", "There's no link").
Fine, I'll just manually create a group message and send it out. And then badger everyone in-person because Facebook is terrible for timely communication.