You and I are both prisoners.

A prisoner’s dilemma doesn’t require actual prisoners; it was the first example of this type of problem, which is why the name is used for all sorts of problems with the same characteristics; namely, that there is some optimal outcome for everyone that could be reached if everyone chose a particular decision, but their is always an individual incentive to ‘defect’ and choose the other option.

Yes I'm aware of that definition, and I still think it is a legitimate example of tragedy of the commons.

The commons here is an abstract one: it's the lack of incentive within a society to perform a kidnapping, which is a shared benefit/resource.

The family of the victim who decides to pay the ransom is taking from this commons (by decreasing the lack of incentive), making everyone worse off in expectation, in order to benefit only themselves and the specific victim they're trying to help.