I’ve read this a few times and I’m genuinely not sure I understand what you’re saying.

4/6th is a smaller ratio than 5/6 the only way for 4/6 to be greater would be for the area to increase.

Expressed as percentages it would be 66% (approx) eaten vs 83% (approx) where the person that ate 66% ate more pizza. The only way that’s possible is if the area of the pizza that 66% of was consumed was greater. (Strictly speaking the volume could be at play here too but I’m going to assume they’re the same height for the question).

I genuinely don’t see any way his thinking was wrong, or how this could be answered another way.

I might genuinely be missing something but if so this question is poorly worded.