It is pretty clear that first a 1024x1024 image is created by DALL-E 3, and then it is outfilled on either side to make it wide. Some images I have gotten back can be cropped to 1024x1024 from wide and you see the entire image in completion, while just beyond there are ambiguous recreations of the content - or nothing.
Also cases where you ask for something like a cartoon with panels at 1024x1024 and it is quite apparently chopped off at the sides - the initial pass leaving something to be interpreted and extended on still.
Therefore, it makes good sense to place this “wide” in the prompt as well, just like it takes clever language to get a tall image consistently rotated.
How about the opposite? Wide language, square specification.
There is definitely a sense that the sides are crowded to be expanded on. Or like the results of the source prompt above, still leave only allowance for non-subject background.