Is cannibalism halal under certain circumstances as per Imam Shafi'i(RA) and Imam Hanbal(RA)?

On a website name Jihadica, various fatwas containing Daesh insignia were collected by certain academicians and researchers in 2014. Fatwa No. 68 is translated by UC Berkeley medical anthropology doctoral candidate, Khashayar Beigi (below): *" the experts of the Shafi'i and Hanbali schools of Islamic jurisprudence permitted killing of the warring infidels or apostates under the constraining circumstances of eating their flesh to stay alive. Imam Alnawawi says, “**There is permission to kill the warring party, the apostate, and moreover to eat their flesh carries no punishment** .” If the experts of jurisprudence have allowed under war conditions believers to eat the flesh of the infidel in order to prevent harm and loss of life, these apply as well to the transplant of organs from the apostate to the Muslim. The life and organs of the apostate are not protected by Muslim jurisprudence."* While the opinions of Daesh certainly do not matter, I want to know whether the opinion they impute on Imam Shafi'i (RA) and Imam Ahmad ibn Hanbal (RA) are accurate or not? Is cannibalism on the kuffar and murtadeen haram or halal?