“According to the Rambam, he would have to say that R’ Hillel was an Apikores. Since Apikorsus is defined as incorrect ideas about Hashem.”
If Hillel was allowed to have those views then at that point it is eileh veeileh divrei Elokim Chayim, as such they are not incorrect.
In addition, I don’t believe apikursis is defined by having incorrect views, rather by having nonpermitted views. I.e. If someone believes that you can use a lemon for an esrog, he is incorrect but he is not an apikores.