“Rambam didn’t include that as an ikkar of faith. Neither did R’Hasdai Crescas. Nor did R’Yosef Albo. It appears in none of the lists of 613 mitzvot.”
Which is why R’ Elyashiv probably did not actually say that such a person is an apikorus and if he did say it, almost certainly didn’t mean it in the halachic sense of the term.