Ed Moloneys Secret History of the IRA was the first book that claimed definitive knowledge of why and how the IRA killed Jean McConville. It claims she was twice found in possession of transmitters and was a spotter for the British Army. In that book Moloney attributed the claim to ibid.
Given the interviews published in Moloneys Voices from the Grave it seems clear ibid, in this case, was Brendan Hughes and he is the only person that has presented this claim of justification for her death.
Until corroboration appears (if it ever does) we only have the word of someone admitting involvement in the killing, legitimising the act. I’d therefore suggest the claim in ‘Secret History’ is not corroboration but repetition from the same source.
Adds – apologies, didn’t put in the root of the ibid. It was ‘Confidential information, IRA source, May 2000.’