First, Goff wouldn't be able to audible out of it. With Skipper announced as eligible, any play they run out of the formation would be illegal formation. I suppose Goff could have re-arranged the line so the formation is correct, basically have Skipper and Decker trade places, but then the whole play is jacked up and you have players out of position.
Second, if the Lions jump up and down and say it's Decker and not Skipper, that blows the discreteness of the play and tips off Dallas on what the play is. The Lions also did not have a timeout. I've also heard once it is announced it cannot be changed. Even so, the ref going and making a correction that it is Decker just draws more attention to the play.