SYSTEM is roughly equivalent to root on linux. My interpretation of the article is that he managed to escalate far enough to get into the kernel, allowing him

SYSTEM is roughly equivalent to root on linux. My interpretation of the article is that he managed to escalate far enough to get into the kernel, allowing him to to modify some data structure to give a user land process higher privileges. So, the overall process could be something like Chrome escape => windows driver vuln => full control of the system.