The blurb about how Lucina came to be is basically "Originally Lucina was a Marth color variation because they're related characters. The have different names and voices but play the same, and would receive treatment similar to male/female WFT, Villager and Robin. However, while Lucina's fighting style is the same, she has different characteristics. Because there's only a small difference, she would be a convenient additional character. This made her a lucky newcomer pick."
or something like that
Maybe there's something on the rest of the page that mentions a cut character, but... I don't see why it wouldn't go along with the section on why she was included. No mention of cut characters or needing to fill in a removed spot.