Jagged frontier is not the same as being benchmaxxed. Benchmaxxed is à la Goodhart's Law "when a measure becomes a target, it ceases to be a good measure." Jagged frontier is about how models that seem superhumanly intelligent at one category of tasks (e.g. coding web applications" can seem toddler level or worse at another category (spatial reasoning) because the training corpus doesn't generalize to there.