wrong: IPCC AR6 puts the warmest Holocene stretch (~6.5 ka) at about 0.2-1°C above 1850-1900. 2011-2020 already beat that
Glaciers don't melt instantly the moment the temperature flips from "form glaciers" to "melt glaciers". They operate on the integral of temperature -- heat. The temperature determines how fast they melt or form.
These melted really, really fast.