This kind of response isn't helpful. He's right to ask about the motivations for the claim that containers in general are "not a sandbox" when the design of containers/namespaces/etc. looks like it should support using these things to make a sandbox. He's right to be confused!

If you look at the interface contract, both containers and VMs ought to be about equally secure! Nobody is an idiot for reading about the two concepts and arriving at this conclusion.

What you should have written is something about your belief that the inter-container, intra-kernel attacker surface is larger than the intra-hypervisor, inter-kernel attack surface and so it's less likely that someone will screw up implementing a hypervisor so as to open a security hole. I wouldn't agree with this position, but it would at least be defensible.

Instead, you pulled out the tired old "education yourself" trope. You compounded the error with the weasely "are considered" passive-voice construction that lets you present the superior security of VMs as a law of nature instead of your personal opinion.

In general, there's a lot of alpha in questioning supposedly established "facts" presented this way.