So if I'm getting this right, they're cancelling do to the fact there have been 8 corona virus cases in the state of california, and are afraid of an outbreak happening at GDC?
It's because it will be a meeting of a bunch of people who've passed through any combination of different international transit hubs to get there. It's not about the known, quarantined, cases that exist in the US right now, but what could sprout any time, and a meeting like this would be a vulnerable situation if someone did come through carrying it.