COMMENT DOWN BELOW IS CORRECT. First we have to understand that we are not talking about 'down below' together. Rather, we are talking about comment down + below. Comment down as a phrasal verb equivalent to write down [which means to write on a piece of paper] has been commonly used to mean leave a comment in the comment section. The word 'below' however refers to the position of where someone leaves their comments. It just so happens that most comments sections are designed to be located under the video, that is, below the video. Had the comment section been positioned at the right side of the video, we could say 'comment down right here or there". The word 'down' doesn't go with the preposition 'below' but should be taken as one unit with 'comment, thus forming a phrasal verb 'comment down'. Therefore, NOT REDUNDANT.