Yes that is true to a certain extent. I would argue though, that Stoicism along with other ancient practices like Taoism teaches sovereignity through true selflessness - not the false altruism or fake solidarity of organized religion and political movements, but a deeper understanding of how the self relates to and interacts with the entire cosmos.
With this understanding, self-improvement is not a worldly achievement but a spiritual one - improving yourself entails realizing both how important and unimportant "you" are, and most of all: what you can and can not control.