It's too early to say whether it works. There's a saying I'm trying to remember, something about 'you can only see the rocks at low tide'. Almost anything will look wonderful in good times, and it's easy to attribute that success to whatever you're doing at the time. It's not until bad times that its flaws become visible.
Zappos was built to its current state on a hierarchical structure. They haven't yet had its decentralized structure tested in any significant way. If Zappos hits a slump in sales for whatever reason, how will the organization respond? Will Holacracy survive a crisis like that? That's where you'll really see if it's a useful system to govern a company.
You can't define 'cult' without an element of normative determination. It's right there in the first sentence on the Wiki page for cult, and in every dictionary definition I could find.
Also, the term does not necessarily carry a derogatory tone. I've seen many small spiritual groups self-describe as cults.
Well, the only difference between a cult and a religion is age. And possibly whether they tell you to see a doctor if you get sick, not sure what the parallel to that would be.
Holacracy a cult? Why not traditional management a cult? The difference being simply that traditional management is broadly accepted as "normal" by society at large.
Ostracism via "the Beach" is just a different kind of social power than the traditional hierachical institutions, with different tradeoffs. Generally you think of these kind of power-diffuse societies as unable to scale, but Zappos has 1500 employees.