Right. The problem is that there are different ways to look at "open." There's nothing stopping anyone from believing other than their own decision. In that sense salvation is open to everyone. Calvinism (and Luther, at least early Luther) looks more carefully at what that decision is based on, and says that only those who God has regenerated can actually make the decision.In that sense salvation is open only to the elect.
The same issue is present with free will. Calvinists believe in free will, in some sense.
So I think from a Calvinist point of view (and I think Luther's) there is a sense in which Christ's death is truly available to everyone and a sense in which it is not. In fact I believe Calvinists and Arminians should agree on the first sense. Arminians do not seem to acknowledge, however, that on another level God determines who is saved, and thus Christ can reasonably be seen as saving only them.
However there is another issue. In addition to the fate of individuals, the NT presents Christ as having a universal scope. There are lots of examples, but the clearest is probably the defeat of Satan. Luke 10:18, Heb 2. Heb 2 is actually a difficult text. While it speaks of Christ as dying for everyone, and Satan as destroyed, it also seems to see the impact on the descendants of Abraham and the children whom God has given you [Christ]. This only works if we see the destruction of Satan as metaphorical, since if Satan is actually destroyed, we're back in Eden. And I think we can agree that that's not our situation. Rather, his universal power is defeated, but that defeat is not yet completely implemented.
My reading of the NT is that Christs life, death, and resurrection had an actual, objective result for the whole world. Still, individuals can reject it for themselves. But the cosmic perspective suggests to me that people who reject God are fighting against what is now the nature of the universe. God doesnt have to condemn them. They condemn themselves.
Although I think both interpretations are possible, I prefer to speak of the atonement as universal, because I think that reflects the cosmic perspective of the NT. But still, the fact that some people exclude themselves is part of Gods plan. That's classic 4-point Calvinism. That's the farthest I'm currently prepared to argue for. As I've noted elsewhere, I still consider open theism a live option (or perhaps semi-open theism, where God controls the overall picture but not all individual actions -- a model consistent with quantum mechanics).
This may well be more consistent with the Eastern view of things than one based on Augustine. The Western Church, following Augustine, as tended to see Christs work in terms of saving individuals from hell. It is based on a concept of pervasive sin that I have started thinking actually contradicts the NT picture of Jesus as victorious. The Eastern Church has tended to see the cosmic aspect of Christ, focusing on his victory over sin and death.
I think this diverges from the original question about Luther. As far as I can tell, Luther and Calvin both operated within an Augustinian perspective, then in discussion of the atonement, Calvin did at times seem to reflect something more Eastern. Here's a discussion of the evidence for Luther:
Beggars All: Reformation And Apologetics: Martin Luther Teaching on Limited Atonement?. As you can see, there's some ambiguity, but he seems to have seen the universal implications of the atonement.