Since there's only one leader, wouldn't it be more appropriate to put an article "the" instead of "a"?
No. This actually changes the context of the sentence. When Will Ferrell says "a world leader," he refers to all the world's powerful countries even though he only mentions the United States. If he said "as the world leader" instead, it would mean that there is only one powerful country on Earth. Ex: Red Cloud was an important leader of the Lakota tribe. => one of many leaders Ex: Neil Armstrong was the first man on the moon. => this means that he was and will always be the first man to have step foot on the moon Ex: As a leader in electric cars, Tesla is quite popular. => this means that they are one of many top companies producing electric cars Ex: As the leader in private space exploration, Space X is well known. => this means that they are currently the best private space company
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