The trend in sea ice extent surrounding Antarctica is actually unclear.  It had been slightly increasing until 2014, but it dramatically decreased in 2016.  Since then it has rebounded toward its long-term average, but scientists are uncertain what this means for the long-term trend.

Obviously, the lack of a clear trend in one physical system does not overturn our understanding that the world is warming.  That understanding is supported by a wealth of data from thermometers, satellites, land ice, movement of crops and habitats, time of budding and flowering, and even the clear downward trend in overall global sea ice.