I’d expect Voldy to know about it and request clarification
Not necessarily. Parseltongue, if I understand it correctly, forces the speaker to tell the truth as he/she understands it (while bypassing Occlumency). If Harry knows about material implication (which he almost certainly does), he can utilize it in such a manner, but it’s unlikely that Voldemort has ever encountered something similar. This isn’t your standard clever wordplay that anyone smart can think of, after all—it’s formal logic, which is decidedly Muggle.
So it’s nonstandard clever wordplay. Voldemort will still anticipate a nontrivial probability of Harry managing undetected clever wordplay. Which means it only has a real chance of working when threatening something that Voldemort can’t test immediately.
I really like “Parseltongue ‘if’ is material implication”, but if this were true I’d expect Voldy to know about it and request clarification, e.g.,
“Explain exactly how they will die, or I will shoot you in five seconds.”
“The world will end if I tell you!”
(admittedly non-optimal)
Not necessarily. Parseltongue, if I understand it correctly, forces the speaker to tell the truth as he/she understands it (while bypassing Occlumency). If Harry knows about material implication (which he almost certainly does), he can utilize it in such a manner, but it’s unlikely that Voldemort has ever encountered something similar. This isn’t your standard clever wordplay that anyone smart can think of, after all—it’s formal logic, which is decidedly Muggle.
So it’s nonstandard clever wordplay. Voldemort will still anticipate a nontrivial probability of Harry managing undetected clever wordplay. Which means it only has a real chance of working when threatening something that Voldemort can’t test immediately.
Correct. I address this in another comment.