"Tankie" is just a pejorative for Marxist, though, like "commie" or "pinko." Marxism is "authoritarian" in that it expressly calls for flipping the Capitalist dictatorship of the bourgeoisie into the Socialist dictatorship of the proletariat, ie turning from a society where the Capitalists are oppressing the working class via the state into a society where the working class wields the state against the Capitalists.
This isn't a real "dictatorship" in the modern sense, but a descriptor for where the balance of power lies, in the working class or Capitalist class, via Public ownership or Private ownership being primary. Socialism is still democratic, but will use the power of the state against the bourgeoisie. All states are authoritarian, what matters is which class is in control of the authority, and how we can move beyond class and thus the state.