I disagree, a computer that is turned off can absolutely be compromised, thought not remotely.
An attacker who gains access to a computer could take the hard drive (generic name for the boot drive or storage drive), then infect it with malware giving the attacker remote access later, then reassemble the machine.
By definition all systems can be hacked, the more complex the system, the more ways it can be hacked.
The one time pad system, is as far as I know the only encryption system that is unbreakable, if used properly, that is because it is so simple in theory, but during actual use in critical times, the system was misused by technicians, enabling some messages to be read.
Network booting with PXE, technically, can be a form of remote boot if its still hard wired to the network , so not even off is safe
To be fair, the PXE boot would not affext a powered off device, it would only be an issue if the device was powered on.
But this is just splitting hairs, you have a good point in that few people would expect a PXE boot attack, not to mention that it could interact with wake on lan, which would turn on a powered down computer.
One time pad system is only unbreakable if you ignore nearly everything about it.
It requires key exchange, storage, cordination, and disposal. In other words, the usual targets.
