I think that the "ABSOLUTELY IRRESISTIBLE" and "ABSOLUTELY UNTHINKABLE" language can be a bit misleading here. Yes, someone with the lesion is compelled to smoke, but his experience of this may be experience of spending days deliberating about whether to smoke - even though, all along, he was just running along preprepared rails and the end-result was inevitable.

If we assume determinism, however, we might say this about any decision. If someone makes a decision, it is because his brain was in such a state that it was compelled to make that decision, and any other decision was "UNTHINKABLE". We don't normally use language like that, even if we subscribe to such a view of decisions, because "UNTHINKABLE" implies a lot about the experience itself rather than just implying something about the certainty of particular action or compulsion towards it.

I could walk to the nearest bridge to jump off, and tell myself all along that, to someone whose brain was predisposed to jumping off the bridge, not doing it was unthinkable, so any attempt on my part to decide otherwise is meaningless. Acknowledging some kind of fatalism is one thing, but injecting it into the middle of our decision processes seems to me to be asking for trouble.