Do you mean the one directly north of it, two tiles up?
Do you think it has to do with it being lined up with all the enterable areas? Because none of the other pyramids are... except for the one you actually do have to enter before going to the castle. I'm surprised it doesn't lead to a duplicate of that pyramid stage, actually...
Yeah, that's the one. Like you said, it's the only one that adheres to the "grid", so I assume that's why it works, to a degree. The rest of the decorative pyramids *not* on the "grid" just glitch up the game. (See below.)
Are there any other off-path areas you can enter using that cheat?
You can "enter" quite a few map objects, some of which you normally couldn't in a normal game, such as the little water holes, the dancing flowers on the world 4 map, and the icebergs on the world 6 map... though most of them either freeze the game, lead to a glitched stage that usually kills you instantly or *also* freezes the game, or, and most bizarrely, lead to a weird "stage" that attempts to use the world map's graphics, but are horribly glitched. You can't do anything in them.
I tested every "enterable" area a long time ago, and *did* find a few that actually *led* somewhere (I recall one of the icebergs in world 6 led to a duplicate of... I think it was 6-8?, and another one led to a hammer bros. stage), though I can't recall the specifics.
Also, as an aside... if you use a Game Genie code that forces you to automatically enter levels, the START panel on world 1, oddly enough, leads to an unused stage.