You are not mistaken in that the topics are (somewhat) related, they all have to do with time. But setting the TZ environment variable doesn't mean your programs don't execute the syscalls discussed in this article.

This is about the speed of execution of the mentioned syscalls, which will be called regardless of the TZ environment variable, and how vDSO changes that. However, by setting the TZ environment variable you can avoid an additional call to stat to as it tries to determine if /etc/localtime exists.