[Ditto]

The quotes I cited both would indicate that this was not a safety, and that this was a touchback.

Wrong. A touchback can only happen on a change of possession -- kickoff, punt or fumble into the opponent's endzone recovered by opponent.

This was a fumble into his own endzone that went out of play... therefore a safety.

[SoothingDave]

A touchback can only happen on a change of possession -- kickoff, punt or fumble into the opponent's endzone recovered by opponent.

Also, a fumble out of the opponent's end zone is a touchback.