No, bad modder, no cookie.
Items are singletons. You want to store data on an item stack, then you need to store that data in the ItemStack.
You should use Capabilities for this, but NBT data is also available.
I learned yesterday(ish) that a certain someone hasn't been doing their last 5% necessary for it to be on Github so that the actual process of updating Forge can begin (where anyone can contribute).
The problem is that the "getHeldItem" methods actually return ItemStack not Item. The method should really be named getHeldItemStackOffHand() instead.
So you simply have to get the Item from the Itemstack, so add .getItem() at the end. like:
playerSP.getHeldItemOffhand().getItem() instanceof ItemBlock