"100% foolproof" is not a realistic goal for any engineered system; what you are looking for is an acceptably low failure rate, not a zero failure rate.

"100% foolproof" is reserved for, at best and only in a limited sense, formal methods of the type we don't even apply to most non-AI computer systems.

Replace 100% with five 9s then. He has a point. You're just being a pedant to avoid it.