Silent, that 10% of the iceberg above the water is displacing all the water that it is going to by adding its weight to the whole iceberg and pressing the 90% below the water downward to factor in the 10% mass above the water.  So, the whole iceberg is displacing all the water that it is going to displace.  AND:  the water contained in this berg, I heard, is more than the water volume of Lake Superior.   No matter how you want to minimize or belittle this event, that is a lot of water.