What I said is that there is no way AI generated "just a recording" of his piece. When I googled, even when I asked ChatGPT, there is no AI presently that can generate an accurate — "1:1" as OP says — mockup recording. If there was, Spitfire would be closing their doors right now.
He also says he prompted it. Okay well, do you actually believe he sat there and typed out an entire note-for-note description and it rendered it? If you believe that, then prompt the AI (he doesn't say which he used) and if you don't get the exact same result, then you know he's lying. I can't get Suno to generate a solo drone, never mind a precise multi-voice harmony for choir that is 1:1 what I'd write.
What has most likely happened here, is he is passing off an AI generated track as "his" work because it fit the "vision" he had or something and is providing you with a transcription. Until someone provides evidence and can replicate this piece with the same prompts or software, there is no reason to believe otherwise.