When we speak of 'hard security boundaries' most people, in this space, are comparing to existing hardware backed isolation such as virtual machines. There are many container escapes each year because the chunk of api that they are required to cover is so large but more importantly it doesn't have isolation at the cpu level (eg: intel vt-x such as VMREAD, VMWRITE, VMLAUNCH, VMXOFF, VMXON).

This is what the entire public cloud is built on. You don't really read articles that often where someone is talking about breaking vm isolation on AWS and spying on the other tenants on the server.