That is true. However, glaciers only grow from the accumulation of snowfall which comes from the constant evaporation of the water in the oceans.
For what it's worth, there are now approx. 138 undersea volcanoes along the western Antarctica with the undersea current which warms the water surrounding the continent..
If sea levels rise, there would also be more area under water. This would produce more evaporation.
If temperatures rise, this would produce more evaporation.
Even if we suppose a huge chunk or the entire glacier plops into the ocean, it would seem that most, if not all, of this additional water would end up back in the atmosphere and/or back into glaciers. I don’t know enough to speculate on a time frame.