Anon, but if chazal are merely transmitting Torah, then their “sayso” is no different than accepting their teachings in any other area – you don’t, for instance, say “chazal say that one should be happy with the amount of money he has, but i disagree, and i don’t think that mishnah in pirkei avos is an important enough part of yiddishkeit that I need to accept”
Once we get into deciding for ourselves what parts of Torah we believe in and what parts we don’t, we’re no longer orthodox. Denying one letter of Torah is denying the entire Torah, because it’s one infinite continuum. Denying a statement of chazal because “well they just say so and you need to prove it to me” is no different than saying “prove to me that ervah is patur from yibum”
Chazal do not have to prove themselves to you. They proved themselves to klal yisroel ages ago and were accepted as the baalei mesorah.
You can’t logically say the two statements of “Not believing it is not denying chazal,” and then say that you deny chazal because it’s just their sayso! That’s a complete contradiction.
And yes, it is important enough to have earned deniers cherem throughout the ages, and was deemed apikorsus openly by the leading poskim.