Omega Defender was a goblin in a mech, so it's not a mech
The spirits are spirits in totems, so they aren't totems
except Piloted Shredder Sneed's Old Shredder or Piloted Sky Golem all have someone inside of them and yet they have the Mech tag. Yes, you only see them when the shredder gets destroyed but imo it's just Blizzard inconsistency.
The reason for that is because Omega Defender refers to the person piloting the mech. The Piloted Shredder, Sneed's Old Shredder, and Piloted Sky Golem are referring to the shredder as the focus of what the minion is. The whole flavor of those three are that they "supposedly" drop the pilot of them when they die and is why they have the mech tag.