PAA: It doesn’t matter. At all. And I’ll explain why.
Whether or not someone is a Kofer is up to HKBH. It has no Nafka Minas to us. So if someone accidentally or intentionally has beliefs that may cost them their Olam Haba, it does not relate to us.
Ah, but what about the fact that a Kofer is Dino K’Goy? That’s only because a Mumar L’Avodah Zarah/K’fira is a Mumar L’chol HaTorah Kula. But as long as they don’t think they’re a Kofer (i.e. as long as they think their views fall in with traditional Torah Judaism) then they’re not Mumrim for Kol HaTorah, so their food, wine, Shechita, etc. are fine. This is quite clear from the beginning of Maseches Chullin.
Ah, but you’ll tell me that Shmuely Yanklowitz and Zev Farber thinks their views are legitimate Torah views. I’ll tell you that they are lying to both you and themselves. They know what they’re saying is K’fira. They just want to redefine K’fira (and Judaism). Which is exactly what K’fira is.