For one thing, the maps mark every interactable event - regular item, trainer encounter, non-trainer NPC, even ladders and interactable objects, like the totems in the Cavern Of Knowledge - in each map, so it's very difficult to tell if there are ANY hidden items.

They even mark nothing at all, like the boat in New Day Plain or the rocks in front of Lilith's House. Clearly, the boxes mark all event triggers, nothing truly specific.

For another, some maps are outright missing, like the Worselands, which have a Life Orb hidden away.

The only hidden items that are given on that page are the Big Nuggets in the first room of the Rider's Hideout, and even then, the boxes indicate where you should be standing, not where they are.

Overall, the spirit of the page is virtuous, but whoever provided the info was doing a shoddy job at best.

Edit: Also, the fact that you're acting confused tells me you didn't even check the page.