As for the rest you're far off base horizons is just a content update like in any other game that adds to the already existing code. Its dlc in the sense that adv packs for dungeons and dragons online are dlc.
It's not just another content update like in any other game because most other games don't involve huge amounts of procedurally generated content. They typically involve a relatively small amount of hand-crafted content (terrain, character models, textures, weapons and other gear, sounds, mission scripts). I'm not trying to belittle what they offer, but they are largely data updates, which are quicker to implement and easier to validate than code changes. A PG universe is going to be all about code changes.
Plus Horizons isn't simply tacking on additional missions/characters/craft/weapons. We're already seeing the mission/reputation/contacts system is being revamped, existing craft have additional physics models (planetary flight), a new craft class (SRV) has been added. It's not *that* common for DLC to revamp the existing game mechanics, in addition to just tacking on new missions/models etc.