I don’t know if the “you” in your comment is “me” or “one,” but to clarify, I personally freely admit to being a post-libertarian, not the real thing, despite my residual libertarian sympathies.
That to one side, you should also read my follow-on piece, “the economic case for a UBI.” In that post, I make the rather limited claim not that a UBI would be perfect, but that it would be more efficient and effective than the income support system we have now. I also would argue that it would require a smaller government than what we have now.
Our current welfare system makes a huge effort to control every detail of the lives of the poor, from what they feed their cats to how they choose to relate to the labor market. A UBI would get rid of all that. It would make the government smaller in two ways: A smaller administrative apparatus (just dishing out money, not snooping in bedrooms and litter boxes), and a smaller degree of control over individual choice.
“Smaller” does not mean “anarcho-capitalist-zero”. I agree, if you could build a world modeled on Galt’s Gulch, you would not need a UBI, or if you did, only a privately organized one.