it's not "underwater ice", it's "ice sitting on soil that is below sea level". The ice sheet is thick enough to be significantly above water level, but it's especially vulnerable to temperature increases in the ocean, since ocean water can start flowing under the ice and melt it from below.

The article is about ice on land melting. But actually when floating ice melts it increases sea levels a little bit as well, as explained here: https://www.quora.com/If-water-is-denser-than-ice-why-do-sea... Note, especially the first answer, which says that the effect from melting floating ice is quite small and that the primary drivers of rising sea levels from global warming are melting ice on land and thermal expansion of sea water.

I hope you understand that an iceberg floating in water displaces the same volume as ice as water [1].

1. I am ignoring the complexity of freshwater and saltwater mixing.

It's not the only flaw in the article. Also it chooses to ignore the fact that unlike the Arctic, the Antartic does actually not have any net ice loss at all; in some parts the ice is melting, but this loss is offset by other parts which are gaining in volume.

https://www.nasa.gov/feature/goddard/nasa-study-mass-gains-o...

Amomgst other things, that ice is acting like a plug for a lot of ice on land.