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AFAIK, they’re talking about those red zones at the base of the Antarctic Peninsula, on the left. These are the outlets of glaciers which until recently had been grounded below sea level, but have recently thinned enough to allow seepage under their base, so that they “suddenly” become floating ice shelves and “Bang! Zoom!” there goes the glacier.

They claim the WAIS (actually, three glaciers feeding from the WAIS, or 5% of the WAIS ice itself) are retreating catastrophically.
Fine.

They claim the “retreating” is due to, or a symptom of, or is occurring simultaneously with, or is measured by a “retreat of the grounding line” towards deeper water. This deeper water is between the original grounding line, and the original continental bedrock at the original shoreline way back upstream of the glacier.

OK, fine.

So … If the moving glacier were melting on the bottom (due to an assumed “warmer water” current getting blown in somehow underneath 450 – 550 kilometers of packed antarctic sea ice!) then the glacier ice would be shallower at least near the tip, right? If not shallower completely across the toe of the glacier underwater, it would be weaker or less consistent and less able to resist the relentless pressure pushing down from the billions of tons of glacier ice higher up (further away from the grounding line), right?

So, if the bottom “toe” of the glacier were melting and were weakening or were disappearing completely, would not that relentless force of the upstream glacier ice FORCE the tip of the glacier to move DOWNSTREAM and further away from the continental sea coast? If it were melting underneath, then it would be shallower, and the glacier would move further UP the shallow slope of the grounding line bedrock?

So, the shallower the glacier ice -> the less ice there is underwater -> for the same force pushing the glacier “downstream” the tip of the glacier must be expanding (getting longer!) or getting pushed further “up” the sloping bedrock underwater -> the further forward the top of the glacier must be observed!

The deeper the glacier ice, the quicker it hits the sloped grounding line underwater, the further BACK the glacier is touching the grounding line bedrock, and the further back the “top” of the glacier appears to be moving.