When did Rav Elyashiv say that not being a young Earth creationist is apikorsus?
As an aside, the whole idea of ikarrim according to the Brisker Rav is that for these beliefs being mistaken doesn’t help, nebach an apikorus is ocheit an apikorus.
I can’t imagine that Rav Elyashiv held that the age of the Earth is one of the ikkarei emunah (none of the Rishonim that I know of include it or anything like it). So not having a mistaken belief about it would not be apikorsus, it would just be a mistake.