Touchback should not be scored as a fumble recovery by the Defense.
On a fumble into the endzone or touchback, no player on the defense is awarded with a fumble recovery in the box scores or statistics. Therefore yahoo should not credit the defense with a fumble recovery. The possession of the ball is given to the other team, the "fumbled" football is not actually recovered by any defensive player.
@Kevin: To calculate the total number of fumble recoveries by the (team) defense, you do not count the total number of fumble recoveries by each individual. Instead, you count the total number of fumbles lost (found in the box score statistics under the heading "FL") by each individual of the opposing team, being careful not to count the same "FL" more than once.
If I play an offensive player along with the DEF of the opposing team, I should expect that any fumble lost by my offensive player to be (unconditionally) offset by a fumble recovery by my DEF.