This doesn't necessarily mean Sana is off the table. They're not saying what Clarke's role will be, only that she will "round out a dynamic cast of characters that Han and Chewie will encounter on their adventures."
For all we know she could be in a supporting role, maybe even an antagonist. She can certainly do an the accent they usually like for Imperial officers.
I think by this point Star Wars has grown out of the idea that they can only have one female character per movie, so there's no reason to just assume Clarke is the love interest.