Or that they develop enough willingness to make an attempt to step back into reality. With any kind of addiction, people do not quit until they are willing to attempt to change their behavior patterns.
Not saying that they would have to log off WoW and go see an IRL therapist. See my suggestion that therapists be made available through in-game methods.
I think that you are referring to a personality disorder rather than the addiction though. While it's true that addiction and isolationist personalities go together in most cases the isolation is a result of the addiction and must be treated as a part of the recovery process.
Alcoholics for instance are often extreme isolationists at least as bad as WoW addicts, perhaps more so since they often don't even have human contact through the medium of the game. They cannot recover however until they develop at least enough willingness to step just far enough outside of their comfort zone to ask for help.
I'm not an expert on addiction and the particulars with MMO addiction may be such that you are correct or that the method employed by the aforementioned therapists will actually be of use. I don't think that enough is known about this specific sub-type of addictive personality at this time to say for sure. However, with what has been discovered with all other types of addictive behavior in the past, the likelihood is high that the same treatment methods and protocol will hold true for this addiction as well. For instance, all recovery programs, for over-eaters, sex addicts, drug addicts, etc. are all based on the program of Alcoholics Anonymous. The specifics may be different but the framework of the recovery process and the necessary "steps" that one must take to recover from those addictions is the same. And it's been the case thus far, until a better method is found, that this same process is the most successful for all of the various forms of addiction.
I will gladly eat my words if I am proven to be wrong. Anything that helps people overcome addiction is great in my book, but I'm not holding my breath on this one. At least they're trying to do something. At the very least their presence in-game may plant the seed of "I might be an addict" into someone's head that will later grow into the willingness to ask for help.