A spin is the result of a sudden aerodynamic imbalance on top of a stall; at least that is one cause of a spin. Every aircraft has its own procedures for recovering from a spin, some a little more touchy than others. I don’t know if this was the case, but it seems an experienced pilot would know how to recover.
It also very possible that they had a mechanical breakdown in the control mechanisms.
These types of accidents make me sad and angry at the same time. If it is determined to be a spin and no mechanical issues, then I blame the FAA. During pilot training, spins are not required to be introduced nor demonstrated to trainees. Only spin awareness is taught in the ground training phase. The only time spin training has to be performed is for Instructor training. As a matter of fact, you don’t have to demonstrate spin recoveries during your checkride for CFI. Yet how many people have to die from spins before the FAA makes it a requirement?
but it seems an experienced pilot would know how to recover.
I think it depends on how much time (which is based on just how high you are). The report says 3,900 feet which in the great scheme of things is not that high which translates into very little time to react.
I don’t think they had enough altitude to recover.