https://www.reddit.com/r/dndnext/comments/9xr8qt/was_the_demilich_errata_change_a_mistake/
Apparently the demilich section of the MM says this:
"Undead Nature. A demilich doesn’t require air, food, drink, or sleep. So great is a demilich’s will to survive that it always has the maximum number of hit points for its Hit Dice, instead of average hit points."
Whoops.