I was referring to DLC, the main benefit of DLC is hindsight, I imagine sakurai would be a bit miffed after seeing lyn curbstomp lucina in CYL only for him to deconfirm her as an assist.
though I don't think you would hear many complaints if her assist was removed to make way for the vastly superior playable character, though it does make you wonder if that rathalos reveal could point to something, but that would just be crazy speculation on my part.
his name is likely still intended to be black knight in both contexts, in japan they refer to him as dark knight, because his international name is the same as aries' title black knight, but then we have the the dark knight class from awakening and fates which only makes me curious if it was changed internationally or if the had the dark knight class used twice in the series (wouldn't be the first time as the priestess class returned in fates but had nothing to do with the gaiden priestess class, and mist had a unique cleric class in a game that had female priests)
so I hope this helps you understand why it makes sense from a direct translation standpoint, as fire emblem almost always changes names in localisation, and that kids is why kamui and arthur(genealogy?) will never be in heroes.
still though, I do hope that we get more proper names in these cases, I really don't like how eternal bond was changed to Ike's theme, but on the topic of music, I would love a remix of Zelgius the Brave in ultimate, it could be to Zelgius the Brave what Unstoppable Destiny is to The Black Knight Lives.
edit: another thing to bear in mind, in the smash ballot we were asked to describe why we wanted to see this character in smash or why they would be a good pick, so sheer quantity of votes may not be the only thing used to decide newcomers, I doubt that every vote was carefully read by daddy sakurai himself to see what we want for christmas, but I imagine they could have first cross referenced characters who popped up more than others then considered the pros and cons to adding them officially. This also could be a point in the black knight's favor, I remember that when I voted I said that he would have an easy moveset(due to having some good things to pull from) and that smash bros needs more villains.