I think that's exactly it. The groundwork for "complex" features must already be there in some sense, maybe through mutations or hitchhiking. The last mutation is probably like flipping the on-off boolean. Or maybe something dramatic like a keystone. I.e. the parts alone don't do anything, but you have this magical bit flipped that suddenly turns everything on and gives them significance.

But yeah one bit/mutation does not a feature make. Not without some prior stuff to latch on to, in my opinion.