(Though I worry that there's a roughly opposite error: thinking that skill in other domains is like skill in pure mathematics and requires the same kind of intellectual work. The same amount, maybe -- though actually I suspect it varies -- but not necessarily the same kind.)

I was addressing the specific skill of reading carefully and not making assumptions that the author hasn't stated, which is highly relevant to learning in general. I agree that the work that goes into understanding things outside of pure math isn't necessarily of the same type as within pure math.