sam2 (and mdd); Rambam, hilchos teshuvo ,perek 3, halocho 8, “shelosho hen hakofrim batorah”, look at the whole mishne and the kessef mishne, interpreting the gemoro in chelek. Indeed, according to to this mishne and the kessef mishne, to deny torah she baal peh will indeed be considered “kefirah”.
The question would be, are “divrei agggada” included in this? I would consider the dating of the calendar a matter of aggadah and, of course, this will revert to the old question, how much must we accept in aggadah, in contrast to halacha, where we are fully bound by precedents?