These 2 Jewish history profs, say the 5m non-Jewish Holocaust victims is a lie. Is it true or are they Deniers?

Not enough information here to make any determination at all. 2 people, given the large number of scholars who have written about the holocaust must be a minority opinion. We do know that many non-Jews also died. maybe the number is in dispute, maybe not. But you can't make a determination from just these 2 people.