He said it they way someone might inform you that they’re a banker, hippie, or Republican, as if it would somehow explain why they like this or do that. But the fact that he identified as being three dimensional only brought with it more questions. Perhaps he was just saying he fancies 3D movies?
“They are trying to discourage me,” he continued in a low voice. “The one-dimensional people, they’re trying to bring me down!”
Or perhaps he was just being haunted by one-dimensional beings.