“The presidency is not merely an administrative office. That’s the least of it. It is more than an engineering job, efficient or inefficient. It is pre-eminently a place of moral leadership. All our great Presidents were leaders of thought at times when certain historic ideas in the life of the nation had to be clarified.” FDR
This implies that a leader-of-thought is someone who clarifies historic ideas as opposed to someone who generates new ideas. Clarification is key to problem solving. A person who can clearly define a problem utilizing existing understanding — aka historic-ideas — will align stakeholders eager to contribute to the solution.
In FDR’s extended quote, it’s interesting that he contrasts engineering and improving efficiency with moral leadership. Since engineers aren’t inherently immoral, I assume FDR’s meaning is that a leader’s purpose can’t consist only of improving efficiency. A thought leader connects the issue at hand to the larger group’s shared values. There is more opportunity to gain unanimous acceptance and collective contribution amongst a large group of diverse stakeholders when a leader’s rhetoric clarifies the problem and solution with respect to shared values.