No, you made no mistake. The ball IS downed in the end zone in the example you gave. Since a ball that is downed out of bounds within the endzone is a touchback, the ball is rewarded to the other team at the twenty. Perfectly consistent.
My mistake was stating unequivocally that a fumbled ball is always spotted at the point it went out of bounds. This is true when it is fumbled backwards, but not when it is fumbled forward. I was correcting my overly general language, that is all.