It's a good idea to minimize the attack surface you expose on the public Internet. Basically the default assumption should be to consider everything insecure. Cloud vendors by nature take ownership of protecting all their public facing APIs and they're in the business of offering and securing those. That doesn't mean you should open everything to the Internet.

Not only that but cloud providers put their managed Kubernetes behind IAM.

If you've managed to breach that and you've got Amazon or Microsoft convinced that you are a privileged account owner, it would be trivial to modify the infrastructure to make a private API endpoint public or create a bastion host with access.

I'll have to look it up, but there was an unauthenticated exploit against the control plane a while ago. Probably https://blog.appsecco.com/analysing-and-exploiting-kubernete... CVE-2018–1002105

While I agree is likely secure enough, remember that cloud operators such as Aws used to be ok with making s3 buckets public by default, which did cause many issues until they finally changed that (and made it harder to make them public even intentionally).