Deathwing was literally the only connection to Dragons, and he was the only reason the Aspects were involved at all. The entire theme of the expansion was Old God agents, which is why the major raids included Cho'gall and the Twilights Hammer cultists and the Elemental Lords.
Like most expansions, they aren't really just containers of one theme. They contain many themes, from which people choose to interpret what is and is not relevant.
As an argument I made before, Legion at the outset is very Demon themed, but if you actually do all the questing on Broken Isles then you'd realize that the entire Pillars of Creation storyline and all of the villains you meet on the Broken Isles are actually Old God related, not demonic. Xavius, Azshara, the Drogbar King and Deathwing's lair are all Old God themed, not Legion. The only main things demon themed are Broken Shore, the Invasions, and the mid tier and end tier raids. The actual leveling content had relatively little to do with Demons at all.