Rune Graft of the Warp Not affecting offering skills

An offering is not a buff, it is a separate object in the world, like a minion or totem that doesn't actively do anything, and it has its own duration that determines how long that object lasts.
That object gives out buffs to multiple different objects (each of your minions, and maybe also you) while it exists, and those buffs are removed when the object dies, but the buffs themselves do not have any duration - if they did they could expire before the offering. So expiry rate can't affect those buffs because they aren't expiring - they're waiting for the object to remove them when it dies.
An offering is like a spell totem support on an aura skill - the totem has a duration, and the aura it gives out applies a buff to you. But that buff from the aura doesn't have its own duration. Expiry rate modifiers don't apply to that aura buff on you, for the same reason as the offering case - it has no duration of its own, it just gets cleaned up when the totem is removed, whether that's because its duration ran out or some other reason.