Not just diskless, but somehow incapable of flashing the BIOS, rewriting the CPU microcode, and loading new firmware into the NICs and other peripherals.
CPU microcode is volatile. BIOS flash used to be jumper protected on old PCs, and many NICS you could remove the flash if you didnt want them to be bootable.
Used to have some machines with zip flash sockets you could remove while the machine was running (useful for flashing linuxbios aka coreboot in the old days).
