That's an assumption we can't prove yet. For all we know she has the same specials but different customization options.Not sure if it was brought up before, but with Lucina being a carbon-copy clone of Marth moveset-wise, doesn't that make her entirely redundant with the Custom Specials?
Ergo, she's not her own character. Case closed.
Plus unless she's literally an alt of Marth and they take up the same roster slot and have the same moveset, she's her own character. Just because she might be redundant doesn't mean she's not her own character. Look at TL.