Yes, but that doesn't mean that libertarianism promotes authoritarianism. It just shows that people are attracted to authoritarianism and that they abandon their ideals to follow it when given a chance. Having liberty means having the liberty to follow bad ideas contrary to liberty.

Neo-reactionaries are not "libertarian" in any reasonable sense of the word, even if they are an offshoot. You can see a similar thing on the left with the repeated propensity for leftist ideals, which in moderation endorse a great deal of personal liberty, to start becoming illiberal in service of more radical ideology.

So in that sense, no, libertarianism does not imply that kind of authoritarianism: there is plenty of room for the majority to simply refuse the radical extremes.

Whether libertarianism necessarily leads to oligarchy is a reasonable question, though. This requires more argumentation than you've given, though, because oligarchy generally is assumed to wield power normally reserved to the state to perpetuate itself. Libertarianism is generally hostile to that part of it. Are market forces adequate to end up with oligarchy anyway? I don't know.

The reason I don't know is that we've had approximately zero actual libertarian governments, because actual libertarianism is too at odds with human nature: despite various intriging proposals, there are a lot of cases where collective action is absolutely critical for a functioning society, and libertarianism finds such things incredibly challenging to enact.

So, system doesn't work. If it did work, though, I do expect that concentration of capital would be the result, but I am less sure that it would be properly termed oligarchy because of the limitations placed on the power of the wealthy. And I'm quite sure that neo-reactionaries are simply not libertarian but, as you say, post-libertarian, just like post-modernism isn't modernism. That it is an inevitable progression is, I think, far from certain.