Securities feeds are, I suspect, more write-dominant and have a much lower fanout rate. They also don't have the outlier-retweeting-outlier or outlier-tweeting-@-outlier problems.
While their products for investment managers are mostly run off of persistent databases, the trader terminals rely on a high-volume, nebulous fan out. For many traders, a five second latency is unacceptable.
Incredibly interesting talk and a good write up. Twitter continues to impress! I was surprised to see Redis playing such a critical role, too.