A global passive adversary is what we are up against. They are not interfering with Tor (that we know of), they simply observe it which is what they are doing. They are a global passive adversary. External assumes they are not a part of the system but this is incorrect, Tor still builds on the same pathway the Internet in general uses are so they are not at all an external threat since they are inherently on the network and can control it on a larger scale if they did wish to.