Not extraordinarily unusual. All you have to do is align the focal plane of the lens with the camera sensor. Usually this is done with a metal flange of greater or lesser complexity. Here he just had to adapt the threads on the lens to the threads on the camera body. Twenty minutes in any well-equipped machine shop.

Modern lenses are very complicated, because they work to cancel out all sorts of subtle and troublesome distortions, but a lens can be very simple indeed. A Lensbaby[1] is just a single lens element attached to some stiff bellows.

And that's it.

You could hand it to Robert Hooke and he would nod in comprehension. He might find the plastics more interesting, but the point is, you could have built a Lensbaby at any point in the last three hundred years.

You can get even simpler than that, as my link notes. Poke a hole in a body cap with a hot needle, and you've made a pinhole "lens", which is thousand year old technology.[2]

1: http://www.dansdata.com/lensbaby.htm for an excellent review. He's actually reviewing a Lensbaby 2, which uses a doublet instead of a single uncoated lens, to reduce chromatic aberration and flare, which is quite missing the point, I feel.

2: http://en.wikipedia.org/wiki/Camera_obscura