Nobody has been to this exact thing before, because this is the first time they’ve done it. Traditionally a ticket to WWDC was paid ($1600), and did include some kind of gift. It has been a jacket for ten+ years, along with enamel pins the last several. But this is a different thing: a much smaller, one-day only event, to which the tickets are free.
I wouldn’t be at all surprised if attendees get something cool, but wouldn’t be too surprised if they don’t, either.