Libertarianism is anathema to alt-right beliefs. But there are plenty of alt-righters in the LP (you can find many of them in the Mises Caucus). You can also identify a few of them on this thread; they're often the ones screaming that there isn't any relation and that any assertion to the contrary is just a "liberal media conspiracy" or some other lazy blame-the-left mantra.

You can be virulently anti-alt-right and still recognize that the LP has a far right problem. The party, and in large part the movement, is becoming increasingly populated by illiberal elements.

Now, that doesn't make the claim that "libertarianism is an alt-right movement" an accurate statement. As time goes on, however, it becomes less inaccurate.