I believe you are correct, and that was my mistake. The problem is that these methods are not that popular and are nothing like the established cryptography functions that are currently in use, so finding information about it is hard. The best thing I could find was an academic paper called "Generalized Scheme For Fractal Based Digital Signature (GFDS)" which proposes a digital signature scheme based on Mandelbrot and Julia fractal sets hence my reply above.