Well, based on this image the miner is placed vertically, not horizontally, which is how most people set up, however, this does not take away from the fact that it could be placed horizontally, i honestly think what that person told you makes no sense.
Sorry, this was my confusion, nevertheless for me vertically means the positions when vents are pointed either upside or downside. Some people build a special shelve stand to secure such position thinking that this will improve the air flow through out Antminer case. But they are wrong. I thought OP meant the same when he asked about vertical, but maybe I was wrong, who knows.