We agree that he is saved IF he is a real believer.
To reject the instruction to be baptized as your first decision as a new believer is to cast doubt on the legitimacy of your believing.
IF the thief on the cross had managed to get down from the cross, he would have chosen to be baptized BECAUSE his faith was real as testified to by Jesus.
But that all it would be is doubt, since we would not know either way.
My impression from the Lutheran view was that they would know for sure.