No, no! You were right the first time. Ralph sits in the FDR chair and angrily tells Lisa "I'm here to play George Washington" for the school play. Lisa gives him an "Uhhhh...Ralph..."
Later when he does the scene he is properly costumed as Washington.
No problem. There is a different scene in a different episode where Krusty is (poorly) playing FDR. It is plausible that scene mixed with this one in the fog of time, making a memory of some Simpsons character actually playing FDR.