Their definition of the “left” section of their spectrum:
Sources with a Left AllSides Media Bias Rating™ display bias in ways that strongly align with liberal, progressive, or left-wing thought and/or policy agendas.
Even they recognize that liberal and progressive ideas are separate from leftwing ideas, but for some reason have chosen to lump them into a single group. Likely because there are very few leftwing media outlets and none of them have any real name recognition when compared to CNN, MSNBC, etc.