But to be honest, images like these seem really strange, and even without this article I'd suspect something is encoded in building heights etc. Because normal image would never have such "weird" buildings. But nevertheless I guess it's able to encode message in even more subtle ways.

The image is incidental. What Fable is actually doing is generic encoding/decoding of a message into a medium. Ask it to hide a message in a poem and it should try to apply those encodings into the new medium. Fable could use quantized, margin-rich encoding. Robustness or invariance of features depends on the medium, so Table select the encoding and features tailored for such medium.

Let's see if we can stego secret messages into SVG pelicans on bicycles, as a test.

Will be interesting to see agents intentionally hiding this stuff in a way operators can't spot from any human readable output or thinking trace, evading detection. Wonder if this has happened yet, but it seems inevitable.