Earlier quoted context omitted.
I don't think that invalidates their conclusions. There's also Epictetus. That man can hardly be accused of being a spoiled aristocrat.
It discredits them. Imagine the modern version – a privileged upper-middle class person with an expensive upbringing and education talking about stoicism ala Ryan Holiday and hard work. Their version of stoicism is most likely about not worrying of the fluctuations of stock values. An economically disenfranchised man's stoicism is about seeing their health degrade.
Interesting ideas are disproportionately likely to come from people who have a lot of leisure time, as that is a prerequisite for having the time to think about anything.