Meteor Golem killing planeswalkers
Yes, Meteor Golem can target and destroy a planeswalker on the battlefield. The player is a &quot;planeswalker&quot; only in lore terms. As far as game mechanics are concerned, &quot;planeswalker&quot; has a defined meaning, and a planeswalker object is not a player and vice versa.
A planeswalker in the mechanical sense is a type of object, a game entity that interacts with players and other objects:

102.1. A player is one of the people in the game. [..]
109.1. An object is an ability on the stack, a card, a copy of a card, a token, a spell, a permanent, or an emblem.

A planeswalker is not a creature, as you already noted, but its own permanent type alongside creatures and others:

110.4. There are five permanent types: artifact, creature, enchantment, land, and planeswalker.

Therefore, if Meteor Golem can destroy a non-land permanent, then that does include a planeswalker on the battlefield.