I was curious, since they did specify above 6.7 if it really was a stupid prediction (well, the prediction isn't stupid as much as the response to it, I'm sure the scientists involved included all appropriate disclaimers).
There have been 10 6.7 or higher earthquakes in California (or nearshore waters) in the last century.
The first was 1918, a 6.9 centered in San Jacinto. Since then the longest stretch without a 6.7 earthquake in California was 28 years between a 7.7 in
Kern County and a 7.2 in 1980 off shore from Eureka.
Four were experienced between 1989 and 1994 (Loma Prieta, off the Mendocino coast, Landers, and Northridge) and we are now 13 years since the last one.
Looking through the longer historical data it looks like the longest stretch was from 1873 (Crescent City -- though Owens Valley had three in 1872) to 1906 (you've probably heard of that one) or 33 years.
So I won't go so far as saying 'dumb prediction' but certainly obvious.
I'm more interested that they say 30% chance of a 6.7+ on the Hayward Fault, which would probably be just about the worst case scenario for California.