It doesn’t have to be a fundamental of the religion to make the denial of it an apikores (or an edited — as some mods took to calling it).
And I’m sure you must be aware of the myriad of Meforshim who explain that Reb Hillel was obviously not going against the Pesukim and he only meant that the Kibutz Galuos and Binyan Habayis will go straight to Hashem Melech and bypassing the stage of a Melech Basar Vadam. Right?
Let’s also not forget the Gemara’s reaction to his statement. The Gemara is essentially saying that he needs a Kappara. If you say it after this, then you can be at best an ???? ?????.