From what I have seen, the reaction from the "internet pontificators (i.e. the an-cap crowd)" to Milei and his actions has been overwhelmingly positive, and it's not even close.
Speaking as someone who has no particular objections to being called an "anarcho-capitalist" (even though I prefer to call myself a "post-anarcho-warlordist", if precise labels are to be insisted upon - but never mind that), I see no reason for an-caps (or any other kind of libertarians, for that matter) to object to "authoritarian [...] decision-making when it comes to [...] slashing the government". Just the opposite, in fact - I don't know of any an-caps who would complain that Milei isn't being "democratic" or "procedurally pure" enough (and I would think it extremely bizarre if I encountered one). And why should they? They don't regard "democracy" or "procedure" as being valid or legitimate excuses for any government's depredations.
IOW: The government tyrannizing the people is objectionable, but people (such as Milei) "tyrannizing" the government is not.
Or, as I said recently in another thread: