The article is about what LLMs can do when generating SVG images, not bitmaps. I think the reason for that is that LLMs can generate the XML text for SVG directly. The implications for AI safety are different, because there might not be any code involved that we can examine. So LLMs could start exchanging hidden messages without humans knowing about it.

But now that I think about it that way, it would probably be more interesting to see what kind of steganography LLMs would use in ordinary text. There's no reason to involve images.