> I tested it with a sample event and I don't see any way to RSVP without logging into an Apple account. Maybe I'm missing something?

You are. I explicitly created a burner email and invited it to an event.

When I navigated from the invite email I was prompted to sign in which I declined. It then allowed me to join the event after I confirmed with an emailed code.

On joining the event I was able to set my name and send a note.

The way I tested it was to create Share link, then navigate to it in an incognito window on Desktop and try to RSVP. I am still unable RSVP without login. Perhaps it works without login if you explicitly invite a certain email.