Ron Nash doesn't seem to be wrong here in the facts he's presenting, however he's picking his words carefully.
This poor impotent deity faced the possibility that the suffering of his Son upon the cross would bring about the salvation of no one
Could easily be rephrased as
God did not dictate that particular people be saved, but allowed free will to preside and for none to be saved forcibly - just as his Son died on the cross, he left himself vulnerable to the possibility that Jesus' death on the cross would bring about the salvation of no one
I've intentionally taken a tone here which makes it sound like God is being overly noble in choosing to take on this "impotence" that Ron Nash describes. My point here is that while he has the facts right, his statement is misleading in that it appeals to a subjective view of what that God would be like, when the same facts can be portrayed very differently.
As you might assume, there are a number of varieties of Open Theism - here is the Wikipedia summary:
Philosopher Alan Rhoda has described several different approaches several open theists have taken with regard to the future and God's knowledge of it.
Voluntary Nescience: The future is alethically settled but nevertheless epistemically open for God because he has voluntarily chosen not to know truths about future contingents. Dallas Willard was thought to hold this position.
Involuntary Nescience: The future is alethically settled but nevertheless epistemically open for God because truths about future contingents are in principle unknowable. William Hasker, Peter Van Inwagen, and Richard Swinburne espouse this position.
Non-Bivalentist Omniscience: The future is alethically open and therefore epistemically open for God because propositions about future contingents are neither true nor false. J. R. Lucas and Dale Tuggy espouse this position.
Bivalentist Omniscience: The future is alethically open and therefore epistemically open for God because propositions asserting of future contingents that they 'will' obtain or that they 'will not' obtain are both false. Instead, what is true is that they 'might and might not' obtain. Greg Boyd and Arthur Prior hold this position."
"Open Theism" - Wikipedia
The key thing here is that all of the positions described above agree on one thing: God does not know the future (this is what "epistemically open" means). Where these views split is that the "Nescience" camps believe the future is "alethically settled" - that is to say that there is already "truth" about the future now in the form of facts. The "Nescience" camps disagree on whether God chooses not to know these facts, or whether he doesn't know these facts because they can not be known.
On the other hand the "Omniscience" camps believe that the future is unwritten entirely, that there are no set truths about the future yet, whether they'd be knowable or not. The split here appears to be less to do with the nature of God and more to do with facts about the future. Either there simply are none (they are neither true nor false- hence not facts) or they are all "might and might not be true". I can't see any real functional difference between these two personally, it just gives philosophers something to argue about..
I'll see if I can find some actual open theist quotes to back this up, but from what I can see, any variety of Open Theist is logically committed to the fact that when Jesus died, it was not a necessary truth that any single future person would be saved.
The only way you can have God knowing any actual facts about this is through some form of middle knowledge (see - Molinism)
But - again, according to Wikipedia, Open Theists do not believe in this by definition:
...Yet other versions of classical theism hold that even though there is freedom of choice, God's omniscience necessitates God foreknowing what free choices are made (God's foreknowledge is closed). Open theists hold that these versions of classical theism are out of sync with:
1 - the biblical concept of God
2 - the biblical understanding of divine and creaturely freedom
and/or result in incoherence
Again, I'll try and find some actual open theist sources rather than just relying on Wikipedia, and I'll update the answer when I do.
On a side note, Ron Nash has a some free courses/podcasts on Biblical Training on the history of philosophy, and also on apologetics. I don't agree with everything he says but he's usually at least entertaining to listen to, in particular he tends to put on a bizarre "false pride" (in contrast to "false humility")- at least, as far as I can tell he's joking about how brilliant he thinks he is.