I think what the advice really means is don't optimize unnecessarily. Figuring out what's necessary vs. what's frivolous optimization is actually a hard judgment call, so people (often good programmers who want to follow best practices) fall back on not optimizing at all, until they run smack into a brick wall. By then it can be too late.
I too know nothing about the Twitter situation, but it wouldn't be unusual if Blaine had nothing to do with the design decisions that are the root cause of their performance problems, nor if he had been asked to "make it scale" without actually having any power to affect the system in the necessary ways.
You go right ahead and optimize early, often, and from both ends. I don't want to get in the way of a man and his passion for optimized code.
It's a failing on my part for even bringing up premature optimization at all, particularly if I wanted to discuss anything other than premature optimization (or evil). I've been programming for over 20 years...I should have known what would happen.