everyone on the planet who heard that knew it was a lie, including every person who voted for him. They voted for him because they KNEW he was lying not because they thought he was telling the truth.
It's more subtle than that. Trump says so many varied and contradicting things with extremely varying levels of seriousness that any voter could squint at what he said and done, read their hopes and dreams into it and then think he's not being serious about the rest.