I think people are just referring to the cutscenes, where Raven is always known to be using her powers in a way that makes her look like she's truly flying compared to Kat. There's also a sidequest in the game where it makes it clear that Raven has a better handle on her powers compared to Kat. So while Raven does have differences, I could understand why people would wish for them to go the extra mile. Also
we know they could do something like that if Panther/Phoenix Mode are any indication.
I don't really care either way, but it'd be interesting to see.