Now onto their PoC. So they don't have rate limiting on some API requests. That's pretty dumb for a service with a public API, but in my experience, most websites don't limit requests rate, because it's always a "let's toughen up security" after-thought. I remember GAE having some anti-DDoS measures, so they may be relying on that while growing the business.
The bulk registering of user accounts is more serious though and could be easily fixed (to some extent) with a captcha. This may be worthy of a tweet, maybe. Instead, Gibson listed all of SnapChat's APIs, even though most of them were irrelevant to the PoC, and slapped 'Full Disclosure' on it.
This is high-school level security researching. We were finding the same 'exploits' in high school. You could probably find these with any service that's only starting out. Glad to see that's the best Gibson could do. If I were Snapchat, I'd fix the two issues and then thank Gibson for spending the time to create an API page for SnapChat.