Ujm… He does not use the word kefirah. He says that we ought not do it, and says that it’s an import from reform/conservative without any halachik basis in chazal for a celebration or ceremony. He further writes that one cannot do anything in a shul for a bas mitzvah because of kedushas bais hakneses (since it amounts to nothing more than a birthday party).
He does not call it kefirah; chachamim hizharu bedivreichem.