I don't think either communism or libertarianism are inherently authoritarian, although several famous implementations may be (esp. in the former case).

right wing libertarianism ignores the tyranny of capital, in it's entirety. The erosion of liberties is A-ok as long as your financial needs coerced you into "voluntarily" giving them up.

communism always has some period where there's a dictatorship of the proletariat that atttempts to re-shape society and squash inequality. This is typically done through a "state", as the society will have to deal with other surrounding states and their interests in controlling and interfering with their government. As this 'dictatorship of the proletariat' goes on with enemies from the outside and within, the state system solidifies into stronger and stronger authoritarianism. The top members b/c the new elites and the power of this state is never given up.

Communism always shoots for libertarianism (in a left wing fashion) but can never achieve it for a sizeable population or for any significant amount of time, and it always results in authoritarianism that is worse than what preceded it.