lilmod, here is the complete context of the effect of what Rav Nachum Eisenstein shlita publicly stated at the podium in front of a large audience of gedolei haTorah and rabbonim on November 6, 2007:
Also, it was a rabbi speaking to the same audience the previous day prior to Rav Eisenstein that cited Rav Chaim Kanievsky that you mentioned above, it was not simply a claim from Natan Slifkin. Specifically, he said from the podium that Rav Chaim Kanievsky was asked if it is mutar to convert a person who is mikabel ol malchus shamayim but is unwilling to believe that the world is only 5768 years old. (This was almost 10 years ago.) Rav Chaim answered that it is in fact assur to convert such a person.
Rav Eisenstein is readily available to confirm this.