Right. Of a glacier, about 90% of it is underwater, so I suppose you could assume that that 90% will not change anything when it melts, but the other 10% will.

I think what they are thinking is that it's like a glass of ice water, with ice cubes in it. Once you put the ice cubes in it, the water level is at the highest it will be, even once the ice melts. This is because the ice is already in the water.

However, it isn't the same as a glacier.