In fact, the authors specifically say on the very first line of their website that the copy/fail primitive can be used as a container escape. The entire premise of this article is flawed and irresponsible.
Yeah, exactly - that's a pretty big "if", and not how a lot of container automation does things. In particular you'd need to hit the base system, it's no help at all if some application files that the host does nothing with can be hit.
Now, there are some "funky" no-fs things that could be opened and are mmap'able/spliceable (some stuff in /proc/*, no idea what exactly though), but it's not immediately obvious to me how this is a generic container escape.