I honestly doubt that this has anything to do with “the media” systematically attempting to erase Haile Selassie from memory. If the “young Americans” described in the article have never heard of Haile Selassie, I think it’s more likely that they are simply not very well informed about the history and politics of Africa in general.
I can’t see on your profile where you are from, but, being British, I have always taken a strong interest in Africa, as do many younger people I talk to. More than 20 African countries were part of the British Empire, and 19 are members of the Commonwealth, with another half dozen actively seeking membership or eligible for membership. Many British people have spent time in Africa, e.g. in the colonial administration, the armed forces, as missionaries, working on development, or as businesspeople. Conversely, a lot of Africans have moved to Britain, as have a lot of Jamaicans, for whom Haile Selassie is very significant. Many African leaders were educated in Britain, served in the British armed forces, etc.
Therefore, I think it is natural that British people are particularly familiar with leaders such as Jan Smuts, Kwame Nkrumah, Jomo Kenyatta, Seretse Khama, Julius Nyerere, Kenneth Kaunda, Hastings Banda, Milton Obote, Idi Amin, Robert Mugabe, Ian Smith, and, of course, Haile Selassie.
I would guess that young Americans are more likely to be knowledgeable about the history of, for example, Latin America, the Philippines, and Vietnam.