Everyone is diseased then, everyone has a vice in life. It's human nature
It's not about vices. A lot of people make that mistake when it comes to psychiatric conditions and thus we have people out there claiming they have ADHD, OCD etc. without ever being diagnosed and having no idea, really, what they're talking about.
To be diagnosed with such a disorder, the addiction, obsession etc. has to actively hinder your daily life to the point you, or at least the people around you, can clearly notice. It needs to get to the point where you're physically, socially and mentally harming yourself because of your condition (insomnia, losing jobs / falling grades, repetitive strain injuries etc.).
The WHO is not doing this to tag the average dedicated gamer as behaviorally ill.