Well, one argument that could be used is that for example Venezuela is not the Marxist variant of socialism, but it is their own thing called Bolivarian Socialism based on ideas of La Patria Grande and Simón Bolivar, a good comparison is that of Gadaffi's Arab Socialism, which picks up ideas of Marxism (Bolivarian Socialism does this too, of course), Islam and Pan-Africanism/Pan-Arabism. So in that case it doesn't strickly follow the rules of what some people would call orthodox scientific Marxism, since both incorporate ideas that might not be 100% coherent with dialectical materialist analysis of certain issues. Besides, both Venezuela and Libya suffered from great US meddling (both countries suffered coup d'Etats, Venezuela suffered two if I recall correctly, and embargoes), and specifically for Venezuela the embargo was pretty harsh since their economy depended a lot on crude oil export, which they could not carry out as they should. Still, if you look at the numbers, Venezuela has greatly improved in the last couple years even though it still has insane amounts of inflation, it went from 438.12 on 2017 to 65,374.08 and in October of last year it was of 1,588.0, so as you can see going from 65,000 to 1,500 is a huge success, even if it's still extremely high in comparison with other countries.

Regarding Kowloon, I'm sure you can find some excuse, but to put it simply, it was a place of a really small area (2.6 ha (6.4 acres)) but with a really high density (50,000 people and 1,923,000/km2) but at the end of the day was a perfect place for such experiment since you don't have the complicated macroeconomics of not even a small country. The place was ignored by the English government (it was during the Hong Kong colonial time) but at the same time prohibited the Chinese government of the mainland from taking it to its own due to its closed proximity with Hong Kong, and furthermore it was not some place that was, for example, a CIA funded place to destabilise a region. In simple words it was ignored by pretty much any type of governmental entity and left to be as it could. You could argue, though, that the place started as a poor place, without access to basic needs, but if anything after its anarcho-capitalist existence it should have improved, not get worse. Construction was done without any type of care for regulations, medical care was done by whomever who had access to some tools, and life was even worse than stuff like Latinamerican Villas or Favelas, since even stuff like sunlight was not accessible for people living in the lower strata, since due to its small area housing needed to be built vertically. I'm sure people will find excuses, but to me it's a perfect example, funnily enough I've met some an-caps who wholeheartedly saw that place a desirable thing. There is a book by a photographer that has some of the best photos of that place at the time called City of Darkness, I think it was available at LibGen, and also the place was shown in the film Bloodsport featuring Jean-Claude Van Damme.