There are no true mathematical fractals in nature, only approximations of them. But to say that ferns are not even approximating fractals at all is clearly incorrect to me.
Sure, the leaves don't randomly spawn in space, filling the leaf region at various points - they're leaves, they grow like leaves. It's still an iterated function system, the same type of fractal as a Sierpinski triangle. The only difference is it's a limited number of iterations.