Is this site Buddhist if its users routinely claim that parts of the Buddhist canon are not authentic?
It happens, but I think that the answer is clear -- no.
Personally, I think it's a disservice to wrap this kind of claim with the flavor of certainty while providing but a rough sketch of a justification or no justification at all. I suppose it could mislead readers into thinking that the claim is common knowledge, when it's only someone's personal opinion.
But I have no problem if it's explicit that the claim is the author's personal opinion.
I think it's about balance; between repeating dogma, and not talking rubbish.
The question is a bit too confrontational (apologies), and the site can be Buddhist despite not getting e.g. te mahayana, as many devout Buddhists have done so.
I agree with the above though, that without citing someone who claims that X isn't Buddhism, it should definitely be couched as opinion.