I think I mis-categorized Scott Sumner's viewpoint in my posts on expectations here  and here . In this post, Sumner says:
The markets view QE as expansionary. The market monetarist view is the market view. Whenever the market changes its view and becomes more Keynesian, or MMTist, or Austrian, or more new classical, I’ll change as well.
In , this is actually option 3 (instead of option 4):
"This implies that all you need to do is convince markets that Keynesianism is right to make Keynesianism right. Or you could convince markets that monetarism is right, which would make monetarism right."
In  this is the first of the two possibilities (instead of the second):
"First, is that E(x) ≈ T[E(x)] doesn't specify T. In that case ... T ... depends on what humans believe and there is no specific theory of pure expectations (or you just have to convince the market that T = X and it is entirely political ... X could be communism or mercantilism or the Flying Spaghetti Monster). "
This is actually remarkably nihilistic (and basically why I misunderstood Sumner's view). Sumner, an economist, seems to believe there is no fundamental theory of economics. The economy of an alien civilization would most likely work entirely differently from ours.
However, I don't actually believe Sumner has ridden this trolley all the way to the end. It means you can't actually use "economics" to explain why one economic theory failed or another succeeded, e.g. as is frequently done in reference to communism. The reason communism failed in this view is because the economic agents didn't expect it to succeed. Likewise, welfare-state economy works fine if everyone expects it to.
Then why does he tout Reagan and Thatcher? According to Sumner's view, their accomplishment appears to be that they introduced confidence and groupthink to the US and UK. Calling this supply-side reforms is then a misnomer. They have nothing to do with "reform" or the mechanics of supply and demand -- remember, there are no mechanics!
What I really think is happening is something like the two-step of terrific triviality. The strong claim is market monetarism is the optimal theory of expectations. The weak claim is that any theory of expectations is possible.
The strong claim is behind Sumner's view of Reagan and Thatcher -- it matters which theory of expectations is the one to rule them all. The weak claim is behind Sumner's reasons to discount other possible explanations -- sure, your mechanism is plausible (or even observed), but what does the market think? In one case, market monetarism is purely an idea. In the other, it is purely observational.