Its white Christian nationalism, plain and simple. The alliance is born out of racist grievance politics.
The Bible Belt is a bunch of hypocrites. They don't give a sh**t about the morals, its about who is in control and gets to define American culture. And the north isn't becoming more religious, it's becoming more openly racist which has lead to a natural alliance with white people in the south who have mostly never stopped being openly racist.
This is a post that barely has anything to do with the topic at hand and little more than a rant to confirm your ideological priors. It's laughable, for instance, to say the North is more openly racist than it was 30 years ago.
He is correct, on the other hand, about the fact that the unifying force of the party is Christian nationalism and white grievance politics. It's why these two segments of MAGAworld are really part of the same political umbrella that are nowhere near as contradictory as the OP made them out to be. They share a similar bedrock identity of obsessing over the symbolism and trappings of American cultural Christianity and harboring massive, overlapping grievances against cultural outsiders. Rustbelt MAGA and Bible Belt MAGA are really two peas of the same pod.
As America gets more secular, I can see the bible belt MAGA types amalgamating into politics how you describe.
Similarity, young more "woke" evangelicals will join the ranks of the left in demanding society address racial justice and male privilege. Even if they keep their mouths shut about abortion and LGBT issues.