I don’t want to put words in his mouth, but I believe he is saying when Libertarianism is taken to its extremes, wherein individuals believe that they do not need to rely on the social bonds that are necessary to cooperate on larger issues, that cant be resolved on an individual basis, this could lead to feelings of isolation and despondency, which could be filled by a despotic charismatic fascist.

I agree that there are many “could”and “leaps” in this reasoning. I don’t think seeking to pursue my individual interests necessitates that I give up on cooperating with others, if that cooperation is also in my best interests. But once again, I think he was arguing about the extremes of libertarianism.