The cave is created by warm water, so the ice has been replaced by the water.
It is common to find areas under the front of glaciers that have been melted away by warm water.
Warm water here means just warmer than the ice.
The thing that's making this news is the scale of the area that has melted away.
This is the situation scientists have been predicting to happen as our planet warms, water advancing inland below the ice and possibly making the glacier move faster.
Ice that is connected to bedrock will create friction making the ice move at a slower pace. When ice is no longer connected to the bedrock under it will loose this friction and possibly speed up the flow.
This might be one of the tipping points making the ice melt at a faster pace.
As fred say in his comment ice moving over bedrock will create friction this friction will create a layer of liquid water lubricating the ice and make the ice flow faster,And when ice melts at the surface of the ice the melt water will run down cracks in the ice and reach the bedrock lowering the friction even more.