They say elsewhere that they were prepared for 100k updates all at once. Which is the worst.

Edit: you're an sre, you probably have more experience planning these things than I ever will. I just can't help but think about what would happen if some trolls realized they could synchronize their updates and bring down the service. (Although based on the infrastructure it doesn't seem possible to cause lasting damage.)

Yeah, I understand what you're saying. It is conceivable that a bunch of people would coordinate their updates. To prevent new-comers from spoiling the fun, they only allowed users with accounts created before /r/place was launched to send updates.