They blew the play dead, so they can't go back and give him a touchdown, which they never signaled. If they gave him the touchdown, then they can go back and review whether he was out, but it doesn't work the other way around.
They did not "blow the play dead" - that's utter made-up nonsense. Barkley had long crossed the goal line before whistles were blown stopping play (as confirmed by the SJ running down the sideline just in front of the play by a few feet and looking directly at Barkley's feet continuing to run with Barkley and the play all the way into the endzone - there was no whistle and the SJ would not have continued to run parallel with Barkley and the play all the way into the endzone had a whistle blown and nonsensical to claim otherwise!). The timing of the whistle blowing the play dead (i.e., after Barkley had long crossed the goal-line) makes the play, the spot, etc...."Reviewable" and the Referee/Crew is utterly INCORRECT that the play was "Not Reviewable" to see when, and if, Barkley ever stepped OB and if so, where he stepped OB (i.e., the "spotting" of the ball). The absurdity of their ruling was demonstrated by the fact that Barkley was CLEARLY IN-BOUNDS at the point on the field they RETROACTIVELY claimed he was OB!