In the scheme of things, this is not an enormous scandal. Trump commits gaffes on all days that end in "y," so this is just one of many.
But this is still important because the president needs to be trusted when he speaks. His reaction to this shows weakness of character and illustrates why nothing he says can be taken at face value.
The words of the president will always matter, and will always be carefully critiqued, especially in times of crisis. Remember when Obama caught grief when he said the police officer "acted stupidly" in arresting a black man trying to get into his own house in Cambridge, Massachusetts 10 years ago. Some might argue that Obama was correct in his assessment of the officer's actions. But he's the president, and he should have been more diplomatic than that.
The role of president carries enormous responsibility, and a lot of that responsibility is to kind of be the "truthteller-in-chief," to keep everyone calm, to be above the fray, to be the steady hand, to be the parent who leads us through difficulties with grace, wisdom, supportiveness, and a commitment to truth.
It shows weakness, not strength, to double down in a situation where you are caught saying something that clearly is untrue. But what does Trump do? He blames the "phony media" again and calls one of the reporters "lightweight" for pointing out the truth. This is not the actions of a leader.