Stoicism was born in Greece, three and a half centuries before Epictetus. The authors this article focuses on, although they are today considered the most important (mainly because they're the only ones whose complete works have survived and thus the most influential), are part of the late phase of stoicism, so I don't know if it's fair to call them its leaders. Marcus Aurelius considered himself a failed philosopher; Seneca wasn't strictly a stoic philosopher, for example he frequently quoted Epicurus in his letters. I look forward to more threads about this wonderful school of thought, but like jleopold, I felt that this particular article didn't do it justice.
I agree that we have to make the most of the stoic philosophers that the turmoil of history have left us. Epictetus and Musonius Rufus, I think, are particularly worth being aware of, as while they are later Stoics, they were professional, dedicated philosophers. Seneca The Younger and Marcus Aurelius are worth being aware of, because they wrote accessible content extolling Stoic ideas. You're right that Seneca quoted from Epicurus, but there was considerable overlap, as well as significant differences, between the two schools. Also, I agree this isn't a particularly breathtaking article, but I'm discovering from conversation elsewhere that Stoicism is a broad church, and different facets interest different people. Some are interested purely in surface impressions, others want to understand the meat and potatoes of the philosophy itself. Hopefully I'll unearth things more of interest, and I absolutely invite others to start adding content on the topic.