When Does 'Unsound Mind' Not Equal to 'Not Guilty'?

I'm having a major headache with this Sec 84 plea. Everyone says it's easy, but honestly, I'm getting confused. See, Ravi kills his mom in a fit of rage, citing that he was suffering from dementia at the time. Now, he pleads guilty under Sec 84, saying 'I wasn't in control of my senses'. But, then my friend argues, 'wait, dementia is a permanent condition, Ravi can't just say he wasn't in control when he committed an act that's permanent evidence - the dead body!' So, does Sec 84 mean he gets acquitted due to 'unsound mind' or does the fact that the act is permanent mean he's not eligible? Pls help someone!