Perhaps that's true, but I wonder about the great timeless speeches that also hold there own as essays. The Gettysburg Address, Churchill's speeches, JFK's inaugural address, Colbert or Jobs or D.F. Wallace's commencement speeches, etc.
Were they so good that they worked in both forms?
Or maybe they weren't good as speeches. I wasn't there for any of them.
Keep in mind that GP's advice isn't aimed towards well-practiced speechwriters and eloquent speechmakers. It's to people who are nervous even about joining Toastmasters.