No, but seriously, I'll be honest with you: I've never seen someone die of clinical brain death on CM13. I've seen it on other codebases, but never on CM13's. I don't know if this is just chalked up to it wasn't really coded in as a function or if this is some bizarre technicality in this specific situation. If I encountered it I'd just feed them a couple ImiAlky pills and consider it a miracle they're alive, but I can't legitimately think of why they'd be alive past 100% brain damage.
Although if I had to guess, I'd say brain damage like that would cause accumulating oxyloss and the marine has enough Dex+ to keep them alive for the conversation.