The "premature optimization" advice is usually taken to mean "worry about performance later". But it can be very hard to make a system performant after the fact, when the important design decisions have long been made and a lot of code has been built on top of them.

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.