A lot of the flyers without wings have other obvious signifiers. The Kamis and balloon pokemon have cloud elements. Doduo and Dodrio are obviously birds even though they lack wings. Mantyke/tine you can decide for yourself whether the fins constitute as wings. Rayquaza doesn't have wings but is quite clearly a dragon capable of flight. Fan Rotom's a fan, so it's able to use a flying type move about wind. The only ones that really stand out are members of the Hoppip line, which are flying only because they're light enough to be carried through the air. Jumpluff especially is obvious.
Sylveon doesn't have anything remotely close to that. Its ribbons can like sway in the wind. That's about the best evidence the flying camp has. They could have made it clearly a flying type without giving it wings. Coat the thing in clouds and it would be fine. Eevelutions are blatant representatives of their given type. Sylveon absolutely does not do that for the flying type.