The libertarian position is often misinterpreted. We do not suggest that cartels will not form. Rather, that cartels are not sustainable. Our position is that they aren’t sustainable, nor are monopolies, without government support. If they are sustainable, then they’re probably market-efficient, and therefore not a cartel in practice -although its pretty difficult to imagine such a thing. The counter argument is that government interference can end cartels more quickly than the market. Although this is both questionable and comes at a very high price: Phone service was a lot better Before the breakup of ATT, and the attack on MIcrosoft was an attack on the desire of a company to give us for free what others wanted money for.
(One concern: I am not confident that the land problem has been solved however – or that it matters. But I think it is arguable that the problem of land cannot be solved without war on one end of the spectrum and restricted reproduction on the other.)
As others have noted, the Phoebus cartel did disappear quickly. So I assume that you were simply confused by the difference between whether cartels are possible or whether they’re sustainable.
https://www.quora.com/What-is-the-libertarian-position-on-the-Phoebus-cartel