> it was using decorators to mutate some global state

It isn't. The original version was doing that, but the "decorator" one wasn't. The data variable is internal to the closure, so different invocations of the decorator would have different data variables.

> didn’t find the closure stuff that insightful

It's used to attach state to a function. Different invocations of the function would have different state. IME, I'd rather use an explicit class for that, but it's useful with decorators.