All we know from the video of the actress is that it can break Ice, not break Rock, and can be Poisoned. Now please tell me, how do you know that this means It is super effective against ice and weak against rock and poison? What if it regularly damage ice, does less damage to rock, and is susceptible to poison? Then it could be flying. And something else I would like to know is why rule out a dual type? Whats more likely a brand new type or a dual type pokemon?
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They said Sylveon is not a type already used by anything in the Eevee line (including Eevee), and they eliminated Fighting, Steel, and Poison as possibilities. The most interesting part, however, was the when they tested Poison they put up a text caption that read, "It's super effective!?", which was only done during that test. Those captions only come up when something of notice is happening, so it strongly implies that whatever Sylveon's type is, it's weak to Poison-type moves. It also means that the Fighting and Ice interaction may not mean much.
Since Grass is the only type that's currently weak to Poison and Leafeon exists, then it has to be a new type if the type chart hasn't been modified. Considering how much of a spectacle they're making out of finding out Sylveon's type, it makes more sense for them to be building up to a new type reveal than just Flying or something.