he can in his other classes
But the Robin we are getting is clearly not one of those other classes. S/he is wearing the tactician costume. Besides, there are no classes that can use Dark Tomes and wields swords, so Robin clearly didn't recast. Further more, if we consider Robin's reclass options justification for moves, Robin has access to every single weapon type, so why not give him/her a Staff?
Simply put, Robin's reclass options are not strong justification for moves. They should have stuck to things s/he has access to as Tactician/Grandmaster, since that's clearly the class Robin is in Smash.
What could she have as her down B instead of nosferatu?
A number of things. An easy one would be Counter, which every Fire Emblem character has. But it could be another move using the sword, or even a move that changes based on which tome s/he is carrying. It's not like Nosferatu was the only thing that could have worked.
Ignis.
Powers up the next attack
Vantage
A fast dash forward. Faster than Robin's max running speed of course.
That aside, Nosferatsu is fine.
Smash is known to let characters do things they normally couldn't do.
I know. One only needs to look at Ness to see that. I'm not upset about Nosferatu, I just personally would have preferred they didn't include it.