All that came out when south vern was released in korea. So ye its an expansion.

Legion raids, chaos dungeons, new guardians and bridge are not coming with this patch. They are coming in May/June most likely if following the roadmap.

south vern litteraly expands the game world.

Yes, as i said you are correct about the literal term of the word expansion, making something bigger. But an Expansion with a capital E is a lot more than that.

shivering isles of oblivion is an expansion

If Shivering isles was released today it would likely be called DLC, as all it did was add a new region and you had to pay for it. But 15 years ago we didn't have todays concept of DLC so they called it an expansion.