Sam, I think you can even go further.
You can say they are choshud to be tofeis any other shittas yochid of questionable interpretation that has irregardless been rejected for thousands of years.
So can you eat their kashrus? Of course not. Maybe they eat bsar of bchalav. (Which is far less radical)
Can you drink their wine? Of course not. Maybe they are oveid avodah zarah with it in a way that they think is muttar.
Can you marry into their families? Of course not, maybe they are miyabeim a tzaras habas.