This kind of things happens all the time in academia. The authors are either constrained by space due to paper limitations or they are too lazy to explain all the little details that go into the algorithm.

I used to do research in computer vision a few years ago and it used to be that people won't publish their code and they purposely won't put in all of the details of the algorithm in the paper. Many of those algorithms were patent pending and I assume the authors were hoping to make some money from the patents. Compared to that, it's a lot better nowadays where most of the popular papers come with published code.