dmidecode was not being installed on non-x86 platforms, causing "show hardware dmi" to fail on arm64.
As far as I can tell, dmidecode was being pulled in as a dependency of libparted2, which only depends on dmidecode for amd64 and i386.
Apart from "show hardware dmi", there are other instances dmidecode is used, such as generating synthetic mac addresses (https://github.com/vyos/vyos-1x/blob/671d012af37729601d3cdfba65beb1daf68229a3/python/vyos/ifconfig/interface.py#L499) and identifying the virtual machine vendor (https://github.com/vyos/vyatta-cfg-system/blob/16e9d71ffbe4153aaaa23b924b2ebf8ec29715db/scripts/install/install-postinst-new#L213)
show hardware dmi sudo: dmidecode: command not found