Under the Executive role of the President, being the Head of State is one such position and obligation he has to maintain. In this role as Head of State, the President of the United States is responsible for representing the state, i.e, the nation of America.
So, in the act of congratulating the engineers at NASA on their successful space mission, he is exhibiting his role as the Head of State. And by that, it means he's representing the whole people of America in congratulating the engineers on their success.
