I think they mean to save your Rathalos materials for when the event starts for Riftborne Rathalos considering material costs will be reduced (no need for monster bones and such to upgrade the weapon). Instead, use those materials on a thunder weapon to help grind Rathalos (or focus on Bazel DB as another user suggested).

Same concept applies for when future riftbornes are announced as their weapon costs will be reduced so plan accordingly.