One of the major differences between the government under the Articles of Confederation (pre-1789) and under the Constitution (1789 onward) was the lack of a unitary executive under the former. "President of the Congress," which John Hanson was for a period, sounds like it might be something akin to the Prime Minister in the current UK system, but it's really more like the modern president pro tempore in the US Senate - there weren't special executive powers associated with the position, it was literally just the person who presided over the debate. Things we consider to be executive actions, like everything else, were handled by the body as a whole.

In other words, the office that George Washington was the first to hold was fundamentally different than other offices the United States had called "president."

Source: Milkis and Nelson, The American Presidency: Origins and Development, 1776-2014.