To quote the post:

If std::fs::remove_dir_all followed symbolic links, they could find a privileged program that removes a directory they have access to (called temp/), create a symlink from temp/foo to sensitive/, and wait for the privileged program to delete foo/

This is a classic confused deputy exploit - the attacker doesn't need access to the system directory, they just need to be able to point the deputy at that directory. The race condition on checking if the directory is a link means it's possible to trick the privileged program into misusing its privileges.