I did say that there is a dovetailing of the logic in that Calvinism has God allowing free choices of men and still remaining sovereign. And I said in the above post that there are similarities in that and open theism
because in both cases God, even though he is sovereign, is indeed responding as it were to men's choices. I admit that some Calvinists, especially those that lean toward hyper-Calvinism don't accept this but since many do, what can I say. And what can you say. They do, and it's easily proven by the arguments even strict Calvinists give when they discuss the story of Joseph and the action of his brothers. They meant it for evil, scripture says, but God turned it around and made it a good thing that the brothers sold Joseph into slavery. But I admit that here you have God responding and it could be argued, adapting to what men freely chose to do in order to still get his overall sovereign plan of eventually saving Joseph's family accomplished.
The difference as I see it is that in open theism you have a more developed vision of this dynamic unfolding of the future than you do with Calvinism where the sovereignty of God is extended to all events everywhere and where His plans are unfolding everywhere and in all events. It is indeed true that Calvinists tend to become very uncomfortable whenever it is suggested that God in any way would ever respond to what we did as that would allow too much autonomous freedom in their minds.
The reason your statement above is not true is simply that the choice is not that you must either hold to open theism or make God the author of sin. If you insist on that you are creating a straw man of a Calvinistic principle and then arguing against that. They don't hold to that.
I don't see why this would matter if the choices were free and according to the nature of the person making the choice. If our own free will leads us to Hades, infallibly, then why is that still not our own doing? If scripture notes, and human history proves, that as a whole we naturally are evil by choice and by our own choosing I don't see why that is God's fault.
Like I said above, in some area there are similarities, but you don't usually see that until you start getting into the ideas of how free will really fits with God's sovereignty and how a future decreed event that involves free creatures with free wills is really unfolding. I do admit that most of us, either because of lack of looking into it or due to lack of mental acuity (and I'm putting myself here too) will either tend to say God either causes everything or God is waiting to see what happens next like the rest of us, neither of which are true. All I am saying is that if you go deeper you will find that Calvinism has answers to these questions that are at least as good as anything I have discovered so far and while I don't agree with all aspects of Calvinism it is far more formidable as a coherent theology than you think.
And personally, I do not think open theology, in some forms is a false doctrine, especially if it is only used in a practical sense where we as humans are dealing with God in real time and truly living in the present moment only. I don't believe any of this discussion is a matter of questioning someone's Christianity or salvation for that matter but it is good discussion if not overdone. We don't want to end up like Talkative in Pilgrim's Progress.