IMO, it is always okay (in fact, good) to test ideas as we encounter them, and always permissible (and often good) to criticise them if they don't measure up.
At the very mundane level, if someone tells me carrots are purple, I'll tell them they are wrong.
When it comes to Buddhism, it's hard to be so categorical. There are a lot of ideas and techniques which are widely respected and can't, therefore, be dismissed as 'wrong' - and differences between schools are high on that list. But it's also true that members of every school do make false, half-true or misleading statements from time to time and those should be criticised/corrected when possible - respectfully and gently, of course, but firmly.
As for karmic consequences, saying that what is true is untrue is lying, which is never a good thing. Steering people away from a good path is also not a good thing to do. But, in the context of what you quoted, assenting to an untruth or half-truth is also not a good thing.
A statement is not entitled to special treatment just because it is made under the umbrella of 'Buddhism'.
I guess it comes back to being careful about what you say: if you do your best to be truthful and compassionate, you'll generally be okay.