GP here. You're right that the final diff, in this case the second-order diff, cannot by itself prove that my final adjustment fixed all of the broken sentence-end links. But I wasn't merely going on that evidence.

The whole point of using a second-order diff was to allow me to reliably carry forth the knowledge gained by my exhaustive review of the prior diff. That exhaustive review told me that there were a dozen broken sentence-end links. And that's how many showed up as fixed in the final, second-order diff: one dozen.

So the prior and final evidence, together, allowed me to be confident that the adjustment worked as intended.