Without seeing their results, see if you can place the current drought in its proper historical context.
Did the last comparable California drought occur before or after these historic events?
800 A.D. is as far back as their tree ring data goes.
Within that whole record, there has not been a California drought as severe as the current one, the scientists say.
As each of the above events happened on or after 800 A.D., that means that they all must have happened more recently than the last comparable drought in California.
So yeah. It is no exaggeration to say that the current drought is unprecedented.