Bukhari 6878:Killing Allowed In Three Instances (Fornication, Murder & Apostasy)
This is authentic. The points mentioned in it have been discussed in detail by Ghamidi Saheb and his associates. The married Zani was supposed to be killed, not because he is married, but because he was a culprit of fasad fi alard. His marriage made his case worse in the time of the prophet because he had done itmam e Hujjah on his people. The punishment for the deniers of the prophet among the polytheists after Itmam e Hujjah was death. Those who accepted Islam and then reverted from it committed the same kufr, so the death penalty was announced for them. It does not apply to ex-Muslims, whose kufr is not known or confirmed, as it is only God who can tell who is actually kafir (denying delibetarly after realizing the truth)