They specialize in domains management for businesses who consider their domain to be _very_ important. Think Google, Amazon, Microsoft, Wikipedia... (all of those are listed as clients on the wiki page)

As in "pay a lot of money", and we'll dedicate someone to your domain who makes sure that "giving a domain to a stranger without any documents" will _never_ happen.

a number of the largest companies that used to be 'clients' of markmonitor have now basically become their own domain registrars and have a direct relationship with ICANN. Amazon for instance. It's curious that google was one and has offloaded it to squarespace.

I'm pretty sure google never used them for their own domains, and the whole markmonitor/squarespace thing was their "google domains" product where they sold registrar services to others. Besides that they also are a registry for .app/.dev and others, but don't sell them via their own registrar anymore.

This is the best approach IMHO if you're a large, extremely valuable company registering a lot of domains.

I want to know this, too. My enterprise clients tend to like using it but that certainly doesn't mean anything.

See other sibling comments to yours, but you basically have named support contacts who would have been the human-in-the-loop ensuring that a situation like OP's can't happen.

I haven't spoken to them in like a decade, but they also offered other monitoring stuff like notifying you of likely phishing registrations, etc. And it's no longer novel now with options like Route53, but they used to be one of the only solutions with proper RBAC/delegation/audit logs.