He made the story up. When confronted with the fact that units didn't convert some of the units from his story his response was essentially "Oh yeah, well of course I always supplemented my units definitions on all my machines with obscure units.".
EDIT: #19 here https://www.ibiblio.org/harris/500milemail-faq.html
It's also entirely plausible that a local definitions file was used to provide the unit that didn't come in the default install. Hence the anachronism is merely "potential" instead of "definite"