This is either a true statement or a false statement. In this case Trump made a false statement and should issue a correction.
You’re right, he should issue a correction.
It should say that the fact that his Electoral College victory prevented a Hillary Clinton presidency makes it the most important Electoral College victory in the history of America.
What part of the word "guess" don't you understand? A guess is not necessarily factually true is it? In fact, a guess can turn out to be true or false when you look at the facts, but the guess itself is true, assuming the person guessing believes in the guess.
No need to waste time with a correction, his statement already caused a couple of people to show how foolish they are - including the media representative who given one chance to ask the President of the United States a question, asked him about how many electoral college votes Barack Obama got. As if that is of importance to anyone, except wimpy media types looking for some issue to talk about.