What I don't support is the title of the article, the SECRET sources of Cthulhu. In the 1920's, theosophy was no secret. It was widespread and well known. You could easily buy books describing theosophy. Indeed, the 1920's was rife with spiritualism. In the time period that Lovecraft was writing, these things were obvious.
Perhaps the FORGOTTEN sources of Cthulhu would be a better name.
I've been thinking about writing an article like this myself, but more in table form comparing and contrasting theosophy with Cthulhu. I feel very confident in saying that the Cthulhu mythos is just garden variety theosophy turned on its head. In theosophy, spiritual beings are preparing us to ascend, teaching us better ways, so that when the next age comes, we become greater. In the Cthulhu mythos, horrible beings hate us, their teachings drive us toward depravity, and when the next age comes, they plan on wiping us out. Their mystical books don't enlighten you, they drive you mad. Our relationship with greater beings is not based on benevolence, but malevolence. Really, with just a bullet point table, you'd see the comparison in sharp relief.
This all makes me think that Lovecraft really hated theosophy. In that, I finally find a point where I agree with the man.