The analogy between farm work and intelligence work breaks down at their transition continuation, which the author missed, I believe.

When the people were were freed from farm labour, due to farm automation, people could find another economic activity which required the other capacity of humans - intelligence.

The transition from muscle-power work to mind-work was meaningful, as both muscle and mind were attributes of humans, creating demand for humans having mind power instead of muscle power.

But the continuation of that transition breaks down. Muscle power -> Mind power -> ???

Those 3 questions marks at the end are what's scaring people, and what is threatening the stability of the transition from mind work. If both muscle power and mond power are not things of value, how would the humanity progress to create value? Who buys it? Why do they buy it? Who hires whom? Why?

The ??? is capital power. If both human mind and muscle have lesser value then what matters is the capital you possess. Can be financial capital or other kinds of capital. Indeed this is the default in many less developed countries.

Will power seems to have become the new differentiator.

Yes, if you look at the examples the author used such as Indian cotton or tobacco as drivers of increased voluntary labor, both of those carried immediate intangible value as status symbols or bragging rights on top of their material value. It further stratified society which led to more people wanting more stuff and... on and on. What is this cognitive surplus value going to bring? I don't get it.

I guess their point is that, for example-- instead of requiring heaps of study and a genius mind to learn to how to reverse engineer complex things over the course of years, their intelligence can go to something that... AI can't do yet?

I mean some argue the 3d is creativity.

But I am sure people had the same question marks back then, for what's comes next

For many here the answer seems to be that everyone will be product manager or a capitalist (one suspects the people saying this are product managers and capitalists).