I thought right at the beginning they not only stated pretty plainly, but also illustrated with a simple enough example, that they are concerned with neither si

I thought right at the beginning they not only stated pretty plainly, but also illustrated with a simple enough example, that they are concerned with neither simple physical limitation nor a simple idea of will, but with the effects of ideas. You are physically free to do something(freedom, by your description), and able to decide to or not(free will, by your description), but there is another govorner which is what you believe or how you reason on all manner of other possibly related topics. You don't do something, and it's not because you aren't free to, nor because you have no free will to imagine it, nor because you don't want to.