Curious that they didn't undergo serious melting during the Mideaval warm period, or the warm period before that.
Note that they aren't truly comparing similar climatic periods, but rather similar CO2 concentrations. We are supposed to presume that they are the same thing, but CO2 doesn't drive the climate. If it did, the Earth would run to the extreme of cold or hot and stay there, as there would be no mechanism to bring it back.
Paleogeologists estimate that long-term ice, such as exists in Greenland and Antarctica, has only existed about 5% of time in earth’s geologic history. There is also some evidence that glaciation has been world-wide in the past, but it has been infrequent. Of course, the location of the continental land masses changes over time, so there is always room for variation.
But it always changes. It’s not just the Medieval Warm Period. It’s also the longer Interglacial Period. 70,000 years ago, where I am now was under a mile thick sheet of ice. The world is a lot longer now than it was then. Man had nothing to do with it. It was all mastodon farts.