I don't think it's on this rock (just checked the guide you linked to).
Check on the left (supposing you have the default camera angle) of the entrance.
I got it. I was looking too closely for my own good. What I did is I went to YouTube to watch a walk through of that section. I saw that the video showed the switch more on the corner of that same rock. Thinking that perhaps they randomly move it, I pulled back and sure enough, it's on that rock to the left of the one where it was in the other two examples.
