They prepared for the worst (peak 100k users). The "hope" that those 100k would be spread out was based on the statistical likelihood that these 100k wouldn't line up too much over a 5 minute period.

I didn't follow /r/place that much, but I haven't read any complaints about latency or failures so it looks like they did just fine.

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.

I agree, but if you do fail it means you are doing something right. In most cases I think YAGNI suffices for how much engineering to do.