Iron golem doesn't have an animation for sitting down.
This is inconsistent with other two-legged creatures that walk upright.
Most likely it is WAI, since the golem is not really a living creature, it is created by the player.
Below is the command to reproduce:
/summon minecraft:boat ~ ~ ~ {Passengers:[{id:"iron_golem"}]}/summon minecraft:minecart ~ ~ ~ {Passengers:[{id:"iron_golem"}]}/summon minecraft:pig ~ ~ ~ {Passengers:[{id:"iron_golem"}]}Attachments
Comments 7
Iron golems can't be put into a mount position (using a minecart, horse, pig or boat) without commands so this can't be replicated in survival gameplay for one thing
Most likely it is WAI, since the golem is not really a living creature, it is created by the player.