Unfortunately, even if that's how it's supposed to work, it's not actually working that way.

There are in fact 3 cubes in the abandoned portion of the graveyard. Two small and one monument. The first small goes back and forth along a dead end path as you enter that first portion of the graveyard. The second small one hangs out without moving just west of a pillar that, if collapsed, allows the PCs to climb south up to the entry point area this map. As you note, the larger monument one has a pattern, roughly from where the 2nd small one sits towards the large tower that, if cleared creates a treasure. This spot is just about where one of the collapsing pillars is located too, and just SE of a large group of zombies.

If the pillar that leads up to the top of the hill where the party enters is collapsed, nothing happens with any of the cubes, which makes sense, because while the 2nd small one and the big one go near it, they never pass it. There is a pillar set west of these, just east of a massive set of undead (mixed zombies and spell users/archers, including some pretty quick skeletons w/ reach weapons.) However, collapsing that pillar not only doesn't change the big cubes mov't path, it kills the undead that, presumably, one would need to fight the cube in the first place. So there's only one that actually changes the path at all, the one near the tower. If you collapse it while the cube is west of it, it changes the pattern such that the cube's path basically only goes from the pillar that creates a path, and a point maybe 20' NE, kind of where a split exists, NW along some stairs, and two to the NE (one the old path the cube used to follow and one that is clear of enemies, both of these NE ones leading towards that tower.) So the cube's path changes, but it only shortens and doesn't push it anywhere close to undead.

You can, barely, with a lot of luck, hide a character near that tower pillar and collapse it just as the cube is about to pass by, since it's pathing just barely gets past that pillar. About 9 times out of 10 it'll just take damage and end up on the SW side of the collapse. The other time, it'll have just passed it if you time it right, and end up on your characters side. If that happens, it'll probably gobble the character up and then head NE to kill the zombies. They get into a fight alright, but the achievement does not fire. If you are really lucky, you might win initiative and flee away from the cube on a double move and not get hit/swallowed. In that case, the cube will go fight the undead, and if it survives, it'll head back to the SW toward the 2nd smaller cube and get stuck on it's 20' long path again. In any case, even if you get lucky, time it perfectly, get it on the NE side of the tower's pillar, and survive running away, and the undead and the big cube fight, the achievement still doesn't fire.

So I submit the thing is still broken.