I wonder if they did a black highlight digitally and then printed that, not taking the time to verify that the black was dark enough that it was actually making the text under it unrecoverable.
When scanners shine green light into a page, the kinds of details they reveal is extraordinary. I have been able to recover hidden text this way. The same thing happened when I took a camera snapshot of a page. And used filters to make it gray. Hidden text security feature pops out.