The impression I got about subduction was that it was due to the big bend in the big fault. That due to the relative plate motions this section is no longer pure strike-slip, but involved a but of thrust, and that due to millions of years of thrusting parts of one plate are now over/under the other.

However, looking at the topography it would appear that if my interpretation is correct that it is the Pacific plate that over-rode the North American, thus creating the mountain ranges that are on the west side of the San Andreas.

On the other hand, I also think what the researchers are trying to say is that the assumption that the length of the San Andreas is purely or nearly vertical is not a good assumption. There are other strike-slip faults in the area (and likely the world) that show significant dip. That dip doesn't get there 'by accident'.

Brian