I don't think you can actually reach it in the current version, but you probably could in the initial version that had a wallswim if you went to the very right of x1012y1000 (the last screen of the FCG Temple) before shifting.
I would be interested in knowing how you actually reached this.
That's what I thought was the way of reaching it (or something like that), but I now realize you can't. Sorry, I just cheated there.
Do I have to remove that screen?
but it was patched out to keep consistent lore with an obscure you-broke-the-level secret that says the level is too low-budget for wallswims and voidscreens. ...Seriously.)
How do you get there, if I may ask?
There is one screen missing in this (aside from the you-broke-it screens) that was added in v3: x1009y1005, which you can get by going to the rightmost corner of x1008y1005. Hopefully it was actually in v3, and I didn't upload an incomplete version.
Ohhh, I didn't check that corner in my playthrough. When I saw that screen in the editor, I assumed it was just "unfinished business". I'll add it.