I agree that's what the case play says. But it leads to this anomoly:
B55 grabs A44 by the collar and jerks him backward, buckling his knees. Ball comes flying loose, at which point B55 releases the "runner", who regains his balance and does not fall. No foul.
B55 grabs A44 by the collar and jerks him backward, buckling his knees. Ball comes flying loose, at which point B55 releases the "runner", who falls to the ground as a result of the jerk to his collar. Foul for horse collar.
Exact same action caused the fumble, the only difference is whether the runner falls or not. Doesn't seem right. If the horse collar action (note I didn't say "tackle) causes the fumble, the result should be the same, and right now, it's not.
What this case play says to the defense is: grab him by the collar and j#rk the h#ll out of him. If the ball comes loose, let him go, because unless you pull him DOWN, it's not a foul.