You could maybe build and run User-Mode Linux on FreeBSD, and run that in a jail. That's pretty close to your own interpretation of "Linux kernel in a jail" Bu

You could maybe build and run User-Mode Linux on FreeBSD, and run that in a jail. That's pretty close to your own interpretation of "Linux kernel in a jail" But to be real: that's obviously not what was meant. If someone wants to install and run Ubuntu inside a FreeBSD jail, it is well possible to do that. Nobody really cares if it's running kernel.org code, just that the binaries are running as expected.