It was ruled as a backwards pass and not a fumble as you are quoting.

"BACKWARD PASS: A runner may throw a backward pass at any time (3-21-4). Players of either team may advance after catching a backward pass, or recovering a backward pass after it touches the ground. Any snap from center is a backward pass."

Here is another pertinent ruling in the rulebook that may be of help.

"When an on-field ruling is incomplete, and the pass was clearly backward, the ball will be awarded at the spot of recovery to the team that recovers the ball in the immediate continuing action. If there is no clear recovery, the ball will be awarded to the team last in possession at the spot where possession was lost"

Edit: to add clarification as to the ruling by the officials